What is the sum if the first 22 terms of the following series? 86 + 79 + 72 + 65 +58+

Seems to be an arythmetic sequence
Sn=[n(a1+an)]/2
where
Sn means sum of all terms up the nth term
n=number of terms
a1=first term
an=nth term

so from 86 to the 22th term is from a1 to a22

find teh sequence
miknus 7 each time
an=a1+d(n-1)
an=87-7(n-1)
find 22n term
a22=87-7(22-1)
a22=87-7(21)
a22=87-147
a22=-60

S22=[22(87-60)]/2
S22=[22(27)]/2
S22=594/2
S22=297

the sum is 297