I'm trying to find Unknown Coefficients A and B in the following Identity:

$\displaystyle \displaystyle (2x+a)^2 = 4x^2+12x+b$

So i get:

$\displaystyle \displaystyle 4x^2+4ax+a^2 = 4x^2+12x+b$

$\displaystyle \displaystyle 4a = 12$

$\displaystyle \displaystyle a = 3$

$\displaystyle \displaystyle a^2 = b$

$\displaystyle \displaystyle 3^2 = b$

$\displaystyle \displaystyle b = 9 $

The book says the answers are:

a = 3 b = 7

Am i wrong or is the book wrong? :?