When a function can be written as an equation, the symbol f(x) replaces y and is read as "the value of f at x" or simply "f of x".
This does NOT mean f times x.
When y is replaced with f(x) it is called function notation.
REMEMBER: f(-3) means -3 is your input and you substitute it for x.
f(x) = -3 means that your output is equal to -3 and you set it equal to your function.