For each of the given functions f(x), find the derivative (f^-1)' (c) at the given point c, first finding a = f^-1(c).

f(x) = 4x +6x^15, c = -10 a = ?
(f^-1)' (c) = ?
f(x) = x^2 -13x + 52 on the interval [6.5, inf); c = 16 a = ?
(f^-1)(c) = ?