Here's the question you clicked on:
123lal
Find the sum of the first 38 terms of the arithmetic sequence: 3, 8, 13, 18, 23, ...
Which series is it?
a=3 n=38 d=5 \[sum=n\frac{(2a+(n-1)d)}{2}\]
Its in the AP, because every next term has a constant difference with the preceding term
So @Jonask Formula holds for it , apply and get the result.