Chapter 5 - Exponential Functions - 5.2 Solving Equations Using Exponent Rules - 5.2 Exercises - Page 438: 71

$r=6$; Frank divided by 3 instead of taking the cube root of $216$.

Isolate the variable factor on one side, and then take the $n$th root of both sides to solve for the variable. If the root is even, there will be a positive and negative answer. If the root is odd and the other side is negative, there is only a negative answer. $r^3=216$ $r=\sqrt[3] {216}$ $r=6$ Frank divided by 3 instead of taking the cube root of $216$.