Can't really explain that well, but the rate of change in an exponential function should always be decreasing or increasing. Instead of it being constant like a linear equation.
I.E. Not additive, instead to find it we need to divide.
So, it should be 9. However, It does not seem to be maintained for the values after 3. Oh well!
Please let me know if you want me to add an explanation as to why this is the answer. I can definitely do that, I just wouldn’t want to write it if you don’t want me to :)