You can
put this solution on YOUR website!
Now,

--> confirm, by cancelling the dr terms on the right.
so, we have dV/dr, when r=6 --> find dV/dr.
We also know dr/dt --> 2
So we can find dV/dt.
I do not know what the 24 is though... you say it is dr/dt...the rate of change of r, but that is 2, in the question.
Please check your question.
jon.