Without further details, it looks like the form of a differential equation for $P_m(T)$ like this:
$\frac{dP_m}{dT}=-0.0045P_m$. The solution would be an exponential function:
$P_m(T)=A e^{-0.0045T}$.
You can solve for $A$ because you know that at 20 degrees $P_m(20)=1000$. Then you should be able to solve for other values of $P_m(T)$ as needed.
I hope this helps.