Prove that big-Theta(n) + O(n^3) is a subset of O(n^3).
Note that for this problem, you are proving that the set of functions on the left hand side (LHS) is a subset of the set of functions on the right hand side (RHS). The set on the LHS is algebraic sum of two sets (not the union): an element of the LHS has the form f(n) = f1(n) + f2(n), where f1(n) is in the domain of Theta(n) and f2(n) is in the domain of O(n3).