The radius of a right circular cone is increasing at a rate of 5 inches per second and its height is decreasing at a rate of 3 inches per second. At what rate is the volume of the cone changing when the radius is 50 inches and the height is 40 inches?

I went to get help with this problem and I was advised to differentiate the Volume equation in terms of r and h and plug in the numbers. But I didn't get the right answer. I did this:

Plugged in the numbers and got , which isn't right!

What am I doing wrong?