asymptote problem

Hola! I am working on this asymptote problem and I am confused on which would be correct.

An asymptote for the hyperbola x^2/16 - y^2/4 = 1 is.......

now could it be y=4x or y=x. My books gives a few examples and I am not sure which one goes with this problem so I have tried a few and i got these 2 answers....... who knows though I have been wrong before!

In an hyperbola's equation, you have two squared terms, one in "x" and the other in "y". The slopes of the slant asymptotes will be of the form:

. . . . .± sqrt[whatever is under the "y" part]/sqrt[whatever is under the "x" part]

Eliz.

I got y=2x is that right?