I have a problem that seems really basic, but for whatever reason I keep messing up...

f(r) = 7/(1+r)^2

Find the average value of the function on the interval [1,5]

I let u = 1+r

so du = dx

and integral 7u^-2 du

integrated I get -7u^(-1) and the new interval is [2,6]

-7(6)^-1+7(2)^-1 = 7/3

So the answer I keep coming up with is 7/3 which is wrong evidently........ Can anyone help?