In the theory of relativity, the mass of a particle with velocityvis

where is the rest mass of the particle andcis the speed of light in vacuum. Find the inverse function off.

I've tried to makevthe subject and I got this,

When I square root both sides to getv, should I choose the +ve or the -ve square root?

Thevhere is just the speed and no direction is involved here, am I right?