I have a shape that is a slice of a circle and two lines. I want to combine this into one pizza slice shape. I can't figure out how to do it?

Attaching a picture.

I would convert the portion of circle to path (shift+ctrl+c) enable snapping to cusp nodes and with the node tool (n) drag the center of the circle portion to the intersection of the two lines. You can then delete the two lines.

JAM-session - Just Another Method: Draw a pie shape with the circle tool, no fill, black stroke - convert with Path->Stroke to path - take node tool and select all inner nodes and set them to corner nodes - drag (or establish a node at the rounding if there is no) one node from the former round portion to makes the black shape appear. If you need a white/coloured shape go Path->Break Apart and set color accordingly.