Answer:
Last year we had M students.
This year we have N students.
Now, the question ask "by what percentage the number of students grow".
So we are considerating a growth, this means that we must think that M as the 100%.
Then we have that N - M is the difference between the students of last year and this year.
this means that this year we have N - M more students that last year, so the percentage of growth is:
((N - M)/M)*100%
Now you need to replace the values for N and M.
You also can notice that if N is smaller than M, you will have a negative percentage, this means that instead of a growth, we have a decrease in the number of students.