$\displaystyle \int \frac 4{(1+2x)^3} dx$

$\displaystyle =\int 4(1+2x)^{-3}dx$

$\displaystyle =\frac {4(1+2x)^{-2}} {-2}$

$\displaystyle =-\frac {2}{(1+2x)^2}$

$\displaystyle =- \frac {2}{4x^2+4x+1}$

But the model answer is $\displaystyle =- \frac {1}{4x^2+4x+1}$

Why did I do wrong?!