Step-by-step explanation:
step 1. let l be the length of the rectangle
step 2. l = 2x + 4
step 3. lx = 390 yd^2
step 4. (2x + 4)x = 390
step 5. 2x^2 +4x = 390
step 6. 2x^2 + 4x - 390 = 0
step 7. -4 +- sqr(4^2 -4(2)(-390))/2(2) quadratic formula
step 8. (-4 +- sqr(3136))/4
step 9. (-4 +- 56)/4
step 10. x = 13yd, l = 2x + 4 = 2(13) + 4 = 30yd.
