what do I do to get the answer right?

Question

Find a point c satisfying the conclusion of the Mean Value Theorem for the function f(x)=x^−8 on the interval [1,7]. 

c= ???????????????????

Answer 1

x^2 -8

from1 to 7

f(1) = -7

f(7)= 41

[f(7)-f(1)]/(7-1) = (41+7)/6 = 48/6 = 8

f(c) = 8

c^2 -8 = 8

c^2 = 16

c = sqr16=4

it may help to graph this