How do I factor out:

Question

21 - 4x - x^2



x^2 + 4x - 21 = 0
this is of the form (ax + c)(bx + d) = abx^2 + (c+d)x + cd
here ab = 1 so
c+d = 4
cd = -21
therefore c = 7 and d = -3
(x + 7)(x-3)=0
x = -7 and x = 3