Question 75805
x^2-4x=a
x=7
49-28=a
a=21
x^2-4x=21
x^2-4x-21=0
ac=-21
b=-4
-7*3=-21
-7+3=b
(x-7)(x+3)
x=7 and x=-3
we don't use the seven because we already know seven is one of the values 21-3