I am studying and my friends and I are stuck on one question that has us all thinking.
It goes something like this:
Give an example of three planes that only intersect at $(x, y, z) = (1,2,1)$. Justify your choice. The three planes form a linear system of equations.
Solve this by using Cramer's rule.
If you have time, my friends and I would like a motivated answer :)