A Piece of Sheet Metal, 21 Inches Wide, Is Bent $\quad 9$

A piece of sheet metal, 21 inches wide, is bent to form the gutter shown in the illustration. If the cross-sectional area is 54 square inches, find the depth of the gutter.

A) $\quad 9$ in
B) 6 in
C) $\quad 7$ in
D) 8 in