Find formula for the cost C(xyz)

Question

Find a formula for the cost C(x,y,z) of materials for a closed rectangular box that is x feet wide, y feet long, and z feet tall, if the cost for the sides is $7 per square foot and the cost for the roof and floor is $5 per square foot.

Answer 1

C(x,y,z) = Cost of both width walls + Cost of both length walls + Cost of top and bottom

C(x,y,z) = $7(2(xz)) + $7(2(yz)) + $5(2(xy))

C(x,y,z) = $14xz + $14yz + $10xy