If $G(x)= \dfrac {3 x}{6 +2 x}$, find $G'(a)$ and use it to find an equation of the tangent line to the curve $y=\dfrac {3 x}{6 +2 x}$ at the point $(-8, {\frac{12}{5}})$.

$G'(a) =$

Tangent line: $y=$