Using the notation "a(x) =" corresponds to which of the following?

This is just notation so we can differentiated between multiple different functions instead of saying y=x + 4 we say f(x) = x + 4
 
This is because if we had two functions y = x + 4 and y = x - 6
 
it would get confusing to talk about them, so we use this notation to differentiate between theme easily by making f(x) = x + 4 and g(x) = x - 6
 
we can now talk about them easily and avoid confusion.

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