Answer
a) Plane BFG and plane BFE
b) Because two points form a line and two planes intersect in a line.
Work Step by Step
a) $\overset\longleftrightarrow{BF}$ is part of the right side of the cube and part of the front of the cube. The right and front are each part of a plane. Those planes can be named by any 3 points that make up those sides.
b) Postulate 1.3 tells us the intersection of two planes is a line. Postulate 1.1 tells us that a line can be defined by any two points the line passes through.
