"Primary", "Secondary" equation

A fence is to be built to enclose a rectangular region of 4800 square feet. The fencing material along three sides costs $3 per foot.The fencing material along the fourth side costs $4 per foot.

Find the most economical dimensions of the region. In addition to answering the question, label your “primary” equation, “secondary” equation.

---Could someone help me figure out how my professor got these primary & secondary equations

[primary: C = 6y + 7x ; secondary: xy=4800]

[answer: 64.14 ft by 74.84 ft]