Invariant Mass
As you have already learned, the formula for relativistic energy of a particle is
(1)
In order to calculate the mass of the Z boson we enter into the mass formula the sums of the energies and momenta of the decay particles:
Since we are looking at Z decays into either an electron-positron pair or a muon-antimuon pair the sum is over two particles only, giving
Resolving the parentheses:
You recognize the 
as 
and likewise for particle 2, (from juggling around the terms in formula (1)).
So you see that
which, once cleaned up, gives
Taking the square root leads to the final expression of the Z mass expressed by the masses, energies and momenta of the two decay particles.
You therefore need to know all these quantities in order to calculate the Z mass.