Alan T. answered 04/22/19
Stanford alum and mechanical engineer: No subject too big or too small
This is a relatively straightforward application of solving with substitution.
We know that l = w + 3. We also know that l * w = area and that 2*l + 2*w= 34.
If we substitute w + 3 for l in the perimeter equation, we find that 2* (w+3) + 2*w) = 34. Therefore 2w+6+2w = 34 and 4w = 28. Therefore w = 7 and l = w+3 = 10. Area = l * w = 7 * 10 = 70 yards2.