We have a "square" double loop, meaning that both loops go to completion at n.
So there are n^2 executions of t:=t+i+j.
Assuming two additions per execution of the innermost loop, (i.e. ignoring the implied additions in increment of subscripts), we have 2n^2 additions in all.
Therefore the given code section is of O(n^2), i.e. the highest power (if polynomial) and ignore coefficients.
