I have trouble with related rate word problems; half of the time I don't know if the formula I'm using is the right one.

For example, In one of the hw problems:

A right circular cone has a heighth= 8 ft, and the base radiusris increasing. Find the rate of change of its surface areaSwith respect torwhenr= 6ft.

The textbook provides a formula of the surface area as: (pi) * r * the square root ofr^2 + h^2I know that I have to find dS/dr, but they give me two values, which confuses me

[I'm still trying to learn how to right out problems and formulas using LaTex, I apologize.