Explanation:
the easy straight forward method is to simply sum up the 5 terms.
you know that this symbol just means "sum", right ?
and that the index variable is "running" from 1 to 5.
so, you have to sum up
3×1 + 3×2 + 3×3 + 3×4 + 3×5 = 3 + 6 + 9 + 12 + 15 = 45
now, if you want it done via a summary formula to add the first n terms of an arithmetic sequence, like in this case
a1 = 3
an = an-1 + 3 = a1×n = 3n
then
the sum of the first n terms in an arithmetic sequence is (n/2)⋅(a₁+aₙ).
it simply means : first plus last term added up n/2 times.
why ?
because a1 + an = a2 + an-1 = a3 + an-2 = ...
we get n/2 times such part sums.
so, in our case, n = 5, and we get
(3 + 3×5)×5/2 = (3 + 15)×5/2 = 18×5/2 = 9×5 = 45