Answer :
I think f(x) is 2x², and I think you're trying to ask for 'g' of 'f of x'.
g(x) = 3x
g [ f(x) ] = 3 (2x²) = 6x² .
That's choice-'a'.
g(x) = 3x
g [ f(x) ] = 3 (2x²) = 6x² .
That's choice-'a'.