(x-1)(x+3)-6=0

Solve by quadratic formula.

x^2 - x + 3x - 3 - 6 = 0

x^2 + 2x - 9 = 0

When the square term has a coefficient of 1 and the middle coefficient is even, I always complete the square

x^2 + 2x + 1 = 9+1
(x+1) = 10
x+1 = ± √10
x = -1 ± √10