Mechanical torque from rotor power equation

[bcteel]

In my studies for the PE power exam, I have come across the equation Tm = (p*Pr) / (4*pi*f)
Tm = mechanical torque
p = number of poles
Pr = power delivered to rotor
f = frequency in Hz

And I can not figure out how this equation was derived. Can anybody help explain this to me?

 

[osh1397]

Does this help