An approximate equation for calculating the net thrust of a jet engine is:
Fnet = m(vjfe - va )
where:
m = intake mass flow
vjfe = fully expanded jet velocity (in the exhaust plume)
va = aircraft flight velocity
While the m·vjfe term represents the gross thrust of the nozzle, the m·va term represents the ram drag of the intake. Most types of jet engine have an air intake, which provides the bulk of the gas exiting the exhaust. There is, however, a penalty for picking this air up and this is known as the ram drag.
Obviously, the jet velocity must exceed that of the flight velocity, if there is to be a net forward thrust on the airframe.
However for calculation purposes you can assume thrust equal speed, ergo you can calculate the speed if you know the trust and the bypass ratio or the amount (Mass of the air) going through the engine.
Cheers.
