Answer:
f(4) = 7
Explanation:
The notation for these type of problems is very confusing. Just remember that f(x) is the same thing as y (or some variable). Even if there is a number or something on the inside of the parentheses, it's still just a variable.
In our case
f(a) = 2(a + 4) -9
could be rewritten as
y = 2(a + 4) - 9
the thing on the inside of the parentheses signifies what the equation is dependent on.
so when is says f(4), it just want's you to trade out a for a 4, but the equation is still the same.
f(4) = 2((4) + 4) - 9
Now, just follow the order of operations
f(4) = 2(8) - 9
f(4) = 16 - 9
f(4) = 7