I'm trying to solve the following problem:

Consider a cylinder of radius 5 whose axis is along the z axis.

i) what is the rate of change of f(x,y,z) in the direction normal to the cylinder at the point (3,-4,4)?

In the previous part of the question I was asked to find the grad of f = ln(x^2+y^2) + z but I cannot tie this in. I know that the equation of the cylinder is simply x^2 + y^2 = 25.

Any help would be much appreciated!