Greetings! Lets solve this shall we ?
So, we must find the sum of this arithmetic sequence -1,2,5,8,11,14,17 using the arithmetic sequence formula and Sum of an Arithmetic Sequence formula such that
an = a1 + d(n-1)...........where a1 = -1 and a2 = 2. We can use this information to find the common sum ratio 'd' such that,
a2 = a1 + d(2-1)
2 = -1 + d(1)...........Then add 1 to both sides to get
3 = d
We do not need d to find the sum but i decided to show you how to find 'd' in case you needed to find the nth term.
The Sum Formula yields,
Sn = n(a1 + a7) / 2, where n is the number if terms which is 7 in this case, right ? Then we plug in the first and seventh terms for a1 and a7 such that
S7 = (7)(-1+17) / 2 = (7)(16/2) = (7)(8) = 56
I hope this helped!