Calculate.

a) [[$ \displaystyle\frac {x} {5} + \displaystyle\frac {x} {6} = $]]

b) [[$ \displaystyle\frac {2x} {3} - \displaystyle\frac {3x} {5} = $]]

c) [[$ \displaystyle\frac {3} {x} + \displaystyle\frac {4} {y} = $]]

d) [[$ \displaystyle\frac {x} {y} - \displaystyle\frac {a} {b} = $]]