ANSWER
C. 2x
EXPLANATION
The composition (g o f)(x) is,
[tex](g\circ f)(x)=g(f(x))[/tex]
So, if g(x) = x² + 3, replace x with f(x),
[tex](g\circ f)(x)=f(x)^2+3=4x^2+3[/tex]
Solve for f(x). Subtract 3 from both sides,
[tex]\begin{gathered} f(x)^2+3-3=4x^2+3-3 \\ f(x)^2=4x^2 \end{gathered}[/tex]
And take a square root,
[tex]f(x)=2x[/tex]
Hence, the function f(x) is 2x
