(2x+a)^2 - b
expand, multiply out (2x+a)^2 = (2x+a)(2x+a)
= 4x^2 + 4ax + a^2 - b = 4x^2 - 12x
set terms of left and right sides equal
quadratic terms are equal
4x^2 = 4x^2
linear terms are equal when a =-3
4ax = -12x
a = -12x/4x = -3
a= -3
costant terms are equal when b =9
a^2 - b = 0
b = a^2 = (-3)^2 = 9
b=9
(2x+a)^2 -b
= (2x-3)^2 - 9
UNLESS you really meant
4x^2 - 12x = (2x+a)^(2-b)
but that's most unlikely, yet anything is possible.
still, IF so, there are mutiple solutions such as b=2, a = any real number and x = (3+sqr10)/2
4[(3/2 + sqr10)/2]^2 -12(3/2+sqr10)/2 = 1
= [2(3/2+sqr10)/2 + a]^(2-2) = 1
