Because they want you to get an accurite answer on your math paper and not some messed up number
It's

because this choice made the most since.
Answer:
x= 13.5
Step-by-step explanation:
f(x) = 2x + 9
f(x) = 36
Set the 2 equations equal to each other;
2x + 9 = 36
Subtract 9 from both sides;
2x = 27
Divide both sides by 2;
x = 13.5
<em>Check:</em>
f(x) = 2x + 9
f(13.5) = 2(13.5) + 9= 27 + 9 = 36