my problem I am working on is f(x)= x+(54/x); [6,9]

First I took the first derivative and found that it was f ' (x)= 1-(54/x^2)

I then found f(6) = 15 and f(9) = 15

If I plug those into the formulaf(b)-f(a)I get0this just doesn't make

b-a 3

This just doesn't make sense!! Could someone help me?