Verified answer

It's both

Let f(x) = x

To expand vertically by factor of 3:

y = 3 * f(x) = 3x

To compress horizontally by factor of 3

y = f(3x) = 3x

Interestingly enough, both lines and parabolas have this issue. In both cases, a vertical stretch/compression can be re-written as a horizontal compression/stretch, followed by possibly a horizontal/vertical translation.

Some others will work too, but they're usually special cases. The interesting thing is that _every_ line and parabola gets confused about whether they're doing vertical stretches or horizontal compressions. In some sense, they can't tell which one it is.

x = 2.