How do I set up the problem

a rectangular lot whose perimeter is 350 ft. is fenced along 3 sides.
an inexpensive fencing along the lot's length cost $15 per foot.
an inexpensive fencing along the two sides costs only $3 per foot
the total cost of the fencing along the three sides comes to $1905.
What are the lot's dimensions?

let L = length..............W = width
From perimeter...........2L + 2W = 350..........so.....L = 175 - W
From cost.............15L + 2 x W x 3 = 1905....so, 15( 175 - W) + 6W = 1905
Hence, W = 80 and so L = 95

2L + 2W = 350
L + W = 175
L = 175 - W

1905 = (15* L) + (3 * 2 * W)

1905 = 15 L + 6W

635 = 5L + 2W
350 = 2L +2W

285 = 3L
95 = L

635 = 475 + 2W
160 = 2W
80 = W

L = 95 W = 80