What you need to do is merge the 2 edges. To accomplish this: Press Tab to enter edit mode, then 1 to switch to vertex select. Now, click one of the vertices of the side you want to join, then holding Shift, press the adjacent vertex of the other wall. This will select both vertices. Now, to merge the vertices, press M and choose the option that best suits you:
- Merge at first. This moves the second vertex you selected to the first one you selected, and makes them into 1 vertex.
- Merge at last. This moves the first vertex you selected to the second one you selected, and makes them into 1 vertex.
- Merge at center. This moves both vertices to the spot directly between each prior location. Now, repeat with the other 2 vertices.
