how to find v

substitute V for x , so it will be similar to a math problem that you might be use to, 

M=x²/R    First, we need to get R to the opposite side, so instead of dividing, you multiply.

MR=x²     Next instead of x being squared, to get rid of it you would square root it and you need to do same to both sides....

so the square root of MR = x, replace x back with V and you get square root of MR=v

Solve for v:
M=v^2/R

 

M=v^2/R is equivalent to v^2/R=M:
v^2/R=M

 

Multiply both sides by R:
v^2=M R

 

Take the square root of both sides:
Answer: |  v=sqrt(M R)                   or                       v=-sqrt(M R)