[R-sig-dyn-mod] how to model equation with two differential variable?

Hi there,

Don't know if the subject is reasonable. In the model I am trying to 
solve have a function like:

Pi = a * dPhi/dt + b * dAlpha/dt + c
dPhi/dt = ....
dAlpha/dt = ....

I don't know how to make the model equation for the first equation.

I try to use deSolve package.

Any suggestions? Thanks in advance.

Best,
Jinsong