how do you do function notation

$$If f(x) =4x-7\\
then\\
$f(9)=4*9-7=36-7=29$\\
$f(a)=4a-7$\\
$f(-3)=4*-3-7=-12-7=-19$\\
$f(0)=4*0-7=0-7=-7$\\\\
See, each time the x that was in the function bracket to start with is \\replaced with the new thing in the function bracket.\\
Does that help?$$

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