4x^2 +16x -20 = 0      (divide all terms by four, the GCD)
x^2 +4x -5 = 0             (split 4 into 5 and -1)
x^2 -x +5x -5 = 0         (group like coefficients)
(x^2 -x) + (5x -5) = 0   (factor each group)
x(x-1) + 5(x-1) = 0       (factor out the binomial term x-1)
(x-1)(x+5) = 0               (apply the Zero Product Law)
x-1 = 0  OR  x+5 = 0    (solve each linear equation)
x = 1  OR  x = -5           (candidate solutions, check)
4(1)^2 +16(1) -20 =
4 + 16 -20 = 20 - 20 = 0   (check)
4(-5)^2 +16(-5) -20 =
4(25) -80 -20 = 100 -100 = 0   (check)

Thus the two solutions are:  x={1, -5}
