Given f(x)=x+3f(x) = x + 3f(x)=x+3 and g(x)=2xg(x) = 2xg(x)=2x, which of the following expressions represents (g∘f)(x)(g \circ f)(x)(g∘f)(x)?
2x+32x + 32x+3
2x+62x + 62x+6
2x2+6x2x^2 + 6x2x2+6x
x+6x + 6x+6