Observe that

So to find , square root 81 twice, once to get 9 and a second time to get the answer: 3.

Raising a number to the one-fourth power does not mean that you are square rooting it four times. Instead, it is asking: which number multiplied by itself four times gets you the original number? In other words, which number multiplied by itself four times gets you 81? That number is defined as , or 3.