Finding a cross section of a cube I am given the following cube where points $A,B,C$ lie on the bottom, left and back face respectively.
I know how to find cross sections when the points lie on the edges. But, in this case, I dont know how to start. Is there a useful trick to handle this type of problems? Thanks.

I'm assuming you want the cross-section given by the plane through $A$, $B$, $C$.
If so, first note that the plane normal is in the direction $(B-A)\times(C-A)$, so its equation can be written in the form
$$
(X - A) \cdot \big[ (B-A)\times(C-A)\big] = 0
$$
Intersect this plane with each face of the cube.
