response to 73 (posted below)

Since the rectangle has an area of 6000 square feet and area = length * width, you can make the variable for the length of the rectangle x and the width 6000/x. The cost for the first three sides would therefore be 3.75(6000/x + 6000/x + x) and the cost for the fourth side would be 2x, making the function for the total cost f(x)= 3.75(12000/x +x) + 2x. That simplifies to the answer in the back of the book.