The following information will help us with this problem:
[tex]\sqrt[m]{x^n} = x^{\frac{n}{m}[/tex]
When we use that information in the context of this problem, we can find:
[tex]\sqrt[4]{15^7} = 15^{\frac{7}{4}}[/tex]
Thus, a = 15, b = 7, and c = 4.
