I only need help with the yellow highlighted portion, thank you.

Problem from your homework, number 2 from section 4.5 - The management decided to enclose an 807 square foot area outside the building for displaying potted plants. One side is formed by the external wall of the store, two sides are made of pine boards, and one side of steel fencing. If the pine board costs $7/running foot and the steel fencing costs $4/running foot, determine the dimensions of the enclosure that can be built at minimum cost. Questions I need help with Please show written work for this one. Make sure your solution includes an equation for th total cost of building the structure...something like C(x) = ..., and a reason (algebraic, graphical, or in words) how you know what the dimensions are that lead to minimum cost. Write your answer in a complete sentence, like this The dimensions that lead to the lowest total cost are and the lowest total cost is