Is there an easy way of pushing back the faces on the left part on the screenshot (selected faces) to the place the original faces were extruded from? Result should be like the one the right side. The faces were originally extruded like the ones above before using the knife tool to cut out a triangle.
I normally either: extrude it down, which will be harder to get accurate to the same level as it's floor. Or you can delete these faces and edges, and simply create new vertices and edges at the floor level to get the same result.