Add -5 to each side of the inequality:
-3 ≤ x + 5 ≤ 8
-3 - 5 ≤ x + 5 - 5 ≤ 8 - 5
-8 ≤ x ≤ 3
Since x is an integer, there are 12 possible solutions belonging to {-8, -7, ..., 1, 2, 3}. In the sum of these solutions, the integers -3 to 3 cancel out, and the rest give
-8 - 7 - 6 - 5 - 4 = -30