18. Given the formula $V=\frac{4}{3} r^{2} h$, solve for $r$.

Step 1 ：Given the formula \(V=\frac{4}{3} r^{2} h\), we are asked to solve for \(r\).

Step 2 ：First, we isolate \(r^{2} h\) on one side of the equation by multiplying both sides by \(\frac{3}{4}\). This gives us \(r^{2} h = \frac{3}{4} V\).

Step 3 ：Next, we isolate \(r^{2}\) by dividing both sides by \(h\). This gives us \(r^{2} = \frac{3}{4} \frac{V}{h}\).

Step 4 ：Finally, we solve for \(r\) by taking the square root of both sides. This gives us \(r = \sqrt{\frac{3}{4} \frac{V}{h}}\).

Step 5 ：Therefore, the solution for \(r\) is \(\boxed{r = \sqrt{\frac{3}{4} \frac{V}{h}}}\).