Calculate.

a) [[$ \displaystyle\frac {5} {a} + \displaystyle\frac {2} {a} = $]]

b) [[$ \displaystyle\frac {1} {a} - \displaystyle\frac {1} {b} = $]]

c) [[$ \displaystyle\frac {a} {2} + \displaystyle\frac {b} {3} = $]]

d) [[$ \displaystyle\frac {7} {2b} - \displaystyle\frac {3} {b} = $]]