I met such a question in the sixth grade paper of primary school: find the rules, fill in: 2,6,15 (), 56, (), 141 Urgent need help, I know the answer, but do not know the law, seek, analysis. Thank you

I met such a question in the sixth grade paper of primary school: find the rules, fill in: 2,6,15 (), 56, (), 141 Urgent need help, I know the answer, but do not know the law, seek, analysis. Thank you

(31)(92)
6=2+4
15=6+9
31=15+16
56=31+25
92=56+36
141=92+49
(square increasing)