let the universal set be ℝ, the set of all real numbers, and let a = {x is in ℝ | −3 ≤ x ≤ 0}, b = {x is in ℝ −1 < x < 2}, and c = {x is in ℝ | 6 < x ≤ 8}. find each of the following: (a) a ∪ b