Let:
width=W
length=L=W+40.....prob statement
The formula for perimeter is....
P=2x(L+W)
312=2(W+40+W).....substitute for L
312=2(2W+40)........combine like terms
312=4W+80............distribute
232=4W..................subtract 80 from both sides
58=W....................divide both sides by 4
W=58
Now substitute into expression for L...
L=W+40=58+40=98
...and substitute into the perimeter formula...
P=2x(L+W)
312=2x(98+58)
312=2(156)
312=312......check√, your solution is correct.
ALWAYS CHECK YOUR WORK!
So the field dimensions are 98ft x 58ft.
Norbert J. M.
11/01/15