Answer:
g(x) = f(3x) -15
Explanation:
Let f(x) be the parent function.
If the function is to be compressed by a factor of 3 horizontally, we must obtain the value f(3) when x=1, for example. We can do that by using 3x as the argument of the function:
g(x) = f(3x)
If its graph is to be shifted downward by 15 units, every value produced by the function must have -15 added to it. That is, the shifted function is ...
g(x) = f(3x) - 15
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The attached graph shows an example function compressed horizontally and vertically shifted.