Hello, thank you for taking the time to post your question!
This one looks like a complicated solving process but it’s actually just a simple application of the Product Rule. We have the general equation that Profits = Clients x Profit/Client, so applying the product rule and then plugging in the given numbers gives
dT/dt = (N * dP/dt) + (P * dN/dt)
dT/dt = 61(-12.41) + 631.42(2)
dT/dt = 506.83
So the instantaneous rate of change for his profits is approximately $506.83 dollars per year
I hope that helps get you moving in the right direction! Feel free to reach out if you have any additional questions beyond that :)