Let us consider $$x$$
$$\dfrac {-5}{8} + x = \dfrac {-3}{2}$$
Solving for $$x$$,
$$x = \dfrac {-3}{2} - \dfrac {-5}{8}$$
By taking LCM of $$2$$ and $$8$$ is $$8$$
$$\dfrac {(-3\times 4) - (-5\times 1)}{8} = \dfrac {-12 - (-5)}{8} = \dfrac {-12 + 5}{8} = \dfrac {-7}{8}$$
$$x = \dfrac {-7}{8}\therefore \dfrac {-7}{8}$$ should be added to $$\dfrac {-5}{8}$$ to get $$\dfrac {-3}{2}$$.