An equation is shown below:
-2c = pm2 - n

which equation is solved for m?

If you would like to solve the equation -2c = pm^2 - n for m, you can do this using the following steps:

-2c = pm^2 - n
-2c + n = pm^2      /p
m^2 = (-2c + n) / p
m = sqrt((-2c + n) / p)

The correct result would be the second answer; m = sqrt((-2c + n) / p).