Answer:

(7a - 4b)² + 9(7a - 4b) + 20 = 0

let 7a - 4b = x

x² + 9x + 20 = 0

x² + 4x + 5x + 20 = 0

x(x + 4) + 5(x + 4) = 0

(x + 4)(x + 5) = 0

x + 4 = 0

x = -4

x + 5 = 0

x = -5

replace x with 7a - 4b

7a - 4b = -4

7a - 4b + 4 = 0

7a - 4b = -5

7a - 4b + 5 = 0