You most likely only need the first 3 digits after the decimal point but if you needed more i added them! hope this helps
F(7) means f(x) when x = 7. The x is replaced with 7.
So we'll replace every x with 7 to go from x = 7 to 3-x = 3-7 = -4
Put another way
f(x) = 3-x
f(7) = 3-7
f(7) = -4
Answer: -4
The answer to your Q is 4