There's this equation that I have stumbled whilst working on several exercise questions.

I'm sorry that I post this in image, I've tried to write it down using Latex but it shows Error in preview mode.

The instruction is compare the derivative of P in respect to t with the derivative of P in respect to D. I have tried to solve it but my end calculation shows 9/DT instead of what it is supposed to be in the answer key, which is -D/T. I'm a bit confused here, what I'm doing wrong?

Thank You