I am following an example in a book and it makes a jump that I cannot seem to follow. Please help me understand what steps were taken.

Starting Equation:

Which can be rewritten as:

or if p != 900,

Now heres where I get lost. They state that by the chain rule the left side of Eq. (3) is the derivative of with respect to t, so we have:

Any help is greatly appreciated.

Thanks.