Let there be n students in 1st row.There are (n-3) students in 2nd row, (n-6) in 3rd row etc etc

Proof:(By contradiction)

Let us assume that there are 6 rows.

There for the sum of total number of students = 6n-45(Hope u got this)

But 6n-45=630

Therefore 6n = 675Which is not possible.

Therefore ,n=6 is not possible.

Hope U understood.If not pls reply or Nudge me