Concept:
As a linear equation ax+by=c in two variables x and y can be represented by a line in a two-dimensional rectangular coordinate system, so can a linear equation ax+by+cz=d in three variables x, y, and z be represented by a plane in a three-dimensional rectangular coordinate system. And as two non-parallel lines intersect at a point, so can three mutually non-parallel planes intersect at a point.
