Find the mean value of the function on the given interval y=sqrt(x) ; [16,81]

Find the mean value of the function on the given interval y=sqrt(x) ; [16,81]. I need help with this problem. Please!!!

The average value or mean value of some function y = f(x) on the interval [a,b] = 1/(b - a)*∫f(x) dx from a to b.

So in this problem, the average value is simply:

1/(81 - 16) * ∫√x dx from 16 to 81 = 1/65 * [2*(3^5) - 128/3] = 266/39