CN-LEAGUE 2011 Problem 9

Let $f(x)=|\log(x+1)|$ and let $a,b$ be two real numbers ( $a<b$ ) satisfying the equations $f(a)=f\left(-\frac{b+1}{a+1}\right)$ and $f\left(10a+6b+21\right)=4\log 2$ . Find $a,b$ .