In my grade 11 math course the function $f(x)$ is said to have domain $[-4,\infty)$ and range $(-\infty,-1)$. The question asks that range of the function $y=2f(x)$ be determined. The textbook says "this is a vertical stretch by a factor of 2 so it expands the upper bounds of the range by $2$" and so the answer is $(-\infty,-2)$.
I can see how this works for a linear function for example, but I believe that a vertical stretch would have no impact on the range of several other functions such as a parabola, a square root function, or an inverse function. Thus I am a little bit confused.