Your trying to rearrange the equation with v were the r was.
2( v - h ) / k = r
2( v - h ) k / k = rk
2( v - h ) = rk
2( v - h ) / 2 = rk / 2
v - h = rk / 2
v - h + h = rk / 2 + h
v = rk / 2 + h
