I am having this problem at trying to align these two right triangles together at their hypotenuse. I tried all the align commands but I don't know which is the right one to make those sides to be perfectly aligned together. 

This is better illustrated in the attached file.

In adition to this question there is also how to align the hypotenuse of this triangle in paralell with respect of the bottom edge on the page on Inkscape. Can someone help me with the steps?. Please I am not that savvy with this program so a detailed step by step guide would help me a lot. Thanks. 

I assume you want the 2 h to be congruent lines.

Select a triangle - tap it again - pull.tbe circular arrows at the corner of the bounding box until the h is horizontal.  Do for both.  Align.

Here's one way to do it.  Turn on these snap settings: Snap to cusp node, Snap to guides, Snap to rotation centers.  Draw a straight line along the first  hypotenuse.  Convert this to a guide [Object > Object to Guides] or [shift+g].  Move the second triangle to match the target vertex.  Move the rotation center to the target vertex.  Rotate the other vertex until it snaps to the guide.

@Paddy_CAD

Thanks for that, I get the idea. But there is still one question which has been unattended.

How do I align those hypotenuses in parallel to the bottom of the page. I mean in your example what I can see is that the right triangle's hypotenuse is not horizontally aligned. It is pointing about 30 degrees to the right corner of your screen. What I required is how to also align the hypotenuse as if it were the base. Do you get my idea?. This is why I mentioned to be aligned with respect of the bottom edge of the page or an horizontal line aligned with respect of the bottom edge of the document.

Can you please display an example on how to achieve this please?

Same principle with the help of a horizontally guide:

@Polygon 

Please note that I am running the portable version of Inkscape which is 0.91 r13725 and I cannot find the option of snap to guides. Where is it in this version?. Does it exist a keyboard shortcut to enable this or anything?.

Moreover how did you create that horizontal line?. I mean I know how to draw a line and so on, but how do you make it horizontal with respect of the page?.

Please I'm stuck in your instructions!. When I convert the object into a guide I can't see it clearly. Does it exist a way to better visualize it or make it more thicker? Can you attend these questions?. Anyone please?.

http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Snapping.html

@Chemist116  I simple drag a guide from the ruler section at the top.

And for the snapping it´s this sucker:

@TylerDurden

Thanks for that. This reference manual did helped me a lot for this.

@Polygon 

It is just that the snapping thing was not working on my inkscape. The thing is there is a little icon which controls turning on and off snapping with object centers, text and guides. But finally I made it work. Thanks for adding that it was much needed although in my version it is displayed as text.

Yes I also noticed that dragging from the ruler would give you guidelines. The rest was just easy to follow.

I am wondering, How can you make one guideline to be perpendicular with relative to another?. Is there a trick for that? Can you teach this to me?

Holding Shift key will allow to rotate a guide and snapping will do the rest: