f(x) = x^3 - 5x^2 - 3x
find the mean value theorem to find all numbers c in the interval (1,3)

i got the answer 5 +- (sqrt 16) / 3
am i correct

What is "all numbers c in the interval (1,3)"??
And no, you cannot be correct, whatever c is, since both numbers are not in (1,3)...even if the numbers were they still aren't in the interval (1,3).