What it's telling you to do is take the nth root of something raised to the mth power. Without the m, it would look like this.
[tex] \sqrt[n]{x} [/tex]
Without the n it would look like this.
[tex] x^{m} [/tex]
When they are put together, it looks like this.
[tex] x^{ \frac{m}{n} } [/tex]
