$ \frac {ax+2}{bx+2} = \frac{b}{a}$
$x=?$
$$\frac{ax+2}{bx+2}=\frac ba$$$$\Rightarrow$$$$a^2x+2a=b^2x+2b$$$$\Rightarrow$$$$(a^2-b^2)x=2(b-a)$$$$\Rightarrow$$$$x=-\frac{2}{a+b}, \,\ (a+b\neq 0)$$