Which function grows the fastest for large values of x?

Question

f(x)=1.5x

f(x)=8x

f(x)=3x

f(x)=4x2+3

Answer 1

ax^2 grows faster than kx for any a and k you choose.

or, equivalently,

if a > b,
x^a grows faster than x^b

Answer 2

that makes no sense to me

Answer 3

I think the answer is f=1.5x