a rectangular field is 45 feet wide and is enclosed by 210 feet of fencing what is the area of the field in square feet

Find the length of the field first.  To do this, write a formula for the perimeter, P, of the field:  P = 210 ft = 2(45 ft) + 2L.

Then 2L = 210 - 90 = 120, and L = 60 (feet).

The area of the field is thus WL = (45 ft)(60 ft) = 2700 sq ft