jmc

Let $b$ be the weight of one bowling ball and $c$ be the weight of one canoe. We have that $7b=4c$. Multiplying both sides by $\frac{3}{4}$, we have $\frac{3}{4}\cdot 7b=\frac{3}{4}\cdot 4c\Rightarrow \frac{21}{4}b=3c=84$. Solving this last equation for $b$, we have that one bowling ball weighs $\boxed{16}$ pounds.