Write the equation x^2 + 6x - 40 = 0 in the form (x + a)^2 - b = 0 and then solve for x

(x² + 6x + 9) - 49 = 0
(x + 3)2 - 49 = 0

then to solve

(x + 3)² = 49
x + 3 = +-7
x = -10 or x = 4