Answer
$ \displaystyle \frac{9}{5c}$
Work Step by Step
$\displaystyle \frac{3a+3b}{4c}\cdot \displaystyle \frac{12}{5(a+b)}=$
...factor out 3 in the first numerator,
..12$=3\cdot 4$
$=\displaystyle \frac{3(a+b)}{4c}\cdot\frac{3\cdot 4}{5(a+b)} = \qquad$
... apply property: $\displaystyle \frac{P}{Q}\cdot\frac{R}{S}=\frac{PR}{QS}$
$=\displaystyle \frac{3\cdot 3\cdot 4(a+b)}{4\cdot 5(a+b)c}\qquad$... reduce: $4$, $(a+b)$
$= \displaystyle \frac{9}{5c}$
