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9-39.

Write and expression for the area and multiply that by the cost per square foot.

Area: x2
Cost: 3x2

Write an expression for the area of one side.
Write an expression for the area of four sides.
Write an expression for the cost of four sides.

Area: 4xh
Cost: (1)(4xh) = 4xh

This is the sum of your expressions from parts (a) and (b).

100 = x2h

Substitute 'h' into the equation in part (c).