The given expressions are the results of applying the function f to a value that is being multiplied by -3 or 3.
f(-3x) means that the function is applied to -3x, in this case the output is -3f(x)
f(3x) means that the function is applied to 3x, in this case the output is 3f(x)
It should be noted that the function f(x) could be any type of function and the output depends on the function's definition.