You will have two equations: one for the area and another for the cost.
The secondary equation gives you the are
Lenght times Width = Area , that is : xy = 4800 sqr. feet.
and the primary is the cost:
C = 3$ for (2 equal sides X + 1 side Y) + 4$ for (1 side Y)
You write this as follows:
C = 3(2x+y) + 4(y)
C= 6x + 3y +4y
C = 6x + 7y