What is the sum of the arithmetic sequence 8, 15, 22 …, if there are 26 terms? 2,483 2,485 3,487 3,489

Since the sequence is arithmetic, we have an expression of the form:
 
 Where,
 a1: first term of the sequence
 d: common difference
 n: number of terms
 We look for the common difference:
 
 Then, setting values we have:
 
 Then, the sum of the terms is:
 
 Substituting values:
 
 
 the sum of the arithmetic sequence 8, 15, 22 is:
 Sn = 2483

What do you mean i don’t understand