Yes, that is correct. If she averaged "G" on n tests, then her total score is nG. If she averages "X" on the next m tests, her total score for those tests would be mX so here total score over all m+ n tests would be nG+ mX. Her average over all the tests would be . Set that equal to H and solve for X:
which is what you have.
You could also write that as
showing just how the difference between "H", the desired average, and "G", the current average, influences the needed future average.