You have two equations, but it's clear whether you have two unknowns or three. If m1, m2 and u are all known values then you have two equations in two unknowns (v1 and v2) so can solve for v1 and v2. Apparently you have arrived at a quadratic equation in v1, and so you can solve for v1 by using the quadratic formula. But if u is an unknown as well then this is indeterminate since you need three independent equations in order to solve for three unknowns.