Hi, I have this question that I can't seem to work out.

$\displaystyle f(x) = 6x + 7x^{19} $

I need to find $\displaystyle a = f^{-1}(c)$ where $\displaystyle c = -13$

I can't seem to work out an inverse function.

I tried to say

$\displaystyle f(f^{-1}(-13)) = -13 = 6f^{-1}(-13) + 7(f^{-1}(-13))^{19}$

$\displaystyle -13 = 6a + 7a^{19}$

But how can I solve for a?