To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $b$ and $g$ is $bg$. Multiply $\frac{6}{b}$ times $\frac{g}{g}$. Multiply $\frac{k}{g}$ times $\frac{b}{b}$.
$$\frac{6g}{bg}+\frac{kb}{bg}$$
Since $\frac{6g}{bg}$ and $\frac{kb}{bg}$ have the same denominator, add them by adding their numerators.
