If f(x)=-3x+9, then what does f(13)= ?

Question

If f(x)=-3x+9, then what does f(13)= ?

Answer 1

f(x) means f is a function and x is the input which we then want to output as, in this case, -3x+9 

So if x=13 then the output would be -3 multiplied by 13 plus 9 ..........

-39+9

so the answer is -30 or f(13) = -30

Answer 2

Sophie,

 

This is function notation.  f(x) is read "f of x".  You can think of f(x) as the "y" or the dependent variable when x = x.  So when given a function f(x) = -3x + 9, we can just substitute whatever is enclosed in parentheses for x in the equation.  In your example, we would replace every x in the equation with 13.  So f(13) = -3(13) + 9, or -30.