Theoretically for something to have negative rest mass must mean it has negative rest energy.
E0=M0*v^2
Therefore when rest mass of M0 < 0, any value for velocity v squared is greater than or equal to 0, since √ -1 = i where i ≠ ℝ.
Anyway, negative mass means in my mind that the mass...
w=0.5(z+1/z)
w=0.5z+0.5/z
wz=0.5z^2+0.5
0.5z^2+0.5-wz=0
If you could substitute a value of either w or z you could work out the other using Quadratic Formula
x = (-b +- sqrt(b^2-4ac))/2a
Other than this, I can't see how you solve it since you have 2 unknowns. Either way you have a squared...