So, if you're trying to find the value of f(3), you substitute x=3 into the equation. However, you must be careful.
Which equation in the function do you substitute it into?
2x-9 or 4-x?
You would substitute x=3 into 2x-9.
Why?
See the inequalities at the end of each equation? Those tell you which values you substitute into the equation. Since x=3, you find which inequality it fits under.
It's not x>3 because 3 is not greater than 3. It would be x≤3 because 3 is a value less than or EQUAL to 3.
Now substitute the value x=3 into the correct equation.
2x-9
2(3)-9
6-9
-3
Therefore f(3) = -3.