1. The problem is to find the value of a number or expression that equals 81. 2. Since the user states the answer is 81, let's consider a common algebraic problem: solving for $x$ in $x^4 = 81$. 3. The formula used is to take the fourth root of both sides: $$x = \pm \sqrt[4]{81}$$ 4. Simplify the fourth root: $$\sqrt[4]{81} = \sqrt[4]{3^4} = 3$$ 5. Therefore, the solutions are $$x = \pm 3$$ 6. This means $x$ can be either 3 or -3 to satisfy $x^4 = 81$. 7. If the problem was different, please provide more details for a precise solution.