Which expression is not a perfect square?

D) x^2 + 6x + 4

x^2 + 6x + 9 = (x + 3)^2, 4x^2 - 4x + 1 = (2x - 1)^2, 9x^2 + 12x + 4 = (3x + 2)^2, but x^2 + 6x + 4 would need 9 as its constant. SLO M-09-A-20 · Understanding