The range is the set of all possible output, as you feed the function with all possible inputs. Since our inputs (domain) are -3, 0, 3, our outputs are
[tex]f(-3) = 15+2 = 17\\f(0)=0+2 = 2\\f(3) = -15+2 = -13[/tex]
So, the range is
[tex]\{-13, 2, 17\}[/tex]