What you need to do is <strong>merge</strong> the 2 edges. To accomplish this:
Press <kbd>Tab</kbd> to enter edit mode, then <kbd>1</kbd> to switch to vertex select. 
Now, click one of the vertices of the side you want to join, then holding <kbd>Shift</kbd>, press the adjacent vertex of the other wall. This will select both vertices.
Now, to merge the vertices, press <kbd>M</kbd> 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.