So, i got the following problem:

A closed box with rectangular sides is built according to the following specifications: The top and bottom sides are made of a material that costs 5 dollars per square foot. the four vertical sides are made of a material that costs 3 dollars per square foot. The top and bottom sides are rectangles with sides of length x and y, where 2x = 7y. The total cost of materials is 100 dollars. Find the largest possible volume that such a box can hold.

My functions are:

Cost= 10xy + 12zy = 100

V = xyz

2x = y7

I'm trying to put Volume in terms of one variable, but i cant... am i missing something?