Given f(x)=x2−4x+4+x2+6x+9f(x) = \sqrt{x^2 - 4x + 4} + \sqrt{x^2 + 6x + 9}f(x)=x2−4x+4+x2+6x+9. On which interval is f(x)f(x)f(x) constant?
[−3,2][-3, 2][−3,2]
[2,3][2, 3][2,3]
(−∞,−3](-\infty, -3](−∞,−3]
[3,∞)[3, \infty)[3,∞)