Set up equations like this (keep in mind that perimeter of a rectangle is all sides added up, so two lengths and two widths):
2l + 2w = 340
l = w + 10
Plug (w + 10) in for l in the first equation and solve for w:
2(w + 10) + 2w = 340
2w + 20 + 2w = 340
4w + 20 = 340
4w = 320
w = 80
The width of the field is 80 yd. Plug 80 in for w in either equation:
l = 80 + 10
l = 90
The length of the field is 90 yd.
