4x^2+11x-20=4(x-b)(x+4) найдите b

4x^2+11x-20=4(x-b)(x+4)
D=121-4*4*(-20)=441=21^2
x1=-11-21/8=-4
x2=-11+21/8=5/4
4(x-x2)(x-x1)
4*5/4=5
b=5