How do you simplify 1/3(21m + 27)?

Both $21 m$ and $27$ are divisible by $3$.. so we can do this easily
$\frac{1}{3} \left(21 m + 27\right) = \frac{1}{\cancel{3}} \times {\cancel{21}}^{7} m + \frac{1}{\cancel{3}} \times {\cancel{27}}^{9}$
$7 m + 9$