The sum of the first 15 terms of the sequence an=10n+21

That is
the sum of all the terms from a1 to an when you are summing up n terms is n(a1+an)/2
so

find the 1st and 15th term
1st term is a1=10(1)=21=10+21=31
15th term is a15=10(15)+21=150+21=171

and n=15
15(31+171)/2=15(202)/2=3030/2=1515
the sum of the first 15 term is 1515

Where’s the table? be more specific so we can you