Write down what you are given, what you are required to find, etc.

Given:

$\displaystyle \dfrac{dr}{dt} = 2$

$\displaystyle \dfrac{dh}{dt} = 2$

$\displaystyle V = \dfrac13 \pi r^2h$

Required:

$\displaystyle \dfrac{dV}{dt}$ at h = 8, and r = 6.

You will need to express either r or h in terms of h and r respetively. To find this relation, make a quick sketch of the cone, and you'll see that at the instant asked for:

$\displaystyle \dfrac{h}{r} = \dfrac86$

$\displaystyle h = \dfrac{4r}{3}$

This way, you can use the chain rule:

$\displaystyle \dfrac{dV}{dt} = \dfrac{dV}{dr} \times \dfrac{dr}{dt}$

Substitute h in the formula for V, find the derivative and used the chain rule with what you already know