Confusion is often minimized by using straight SI units in your calculation.
That means amp, not milliamp. 3600s, not hour.

Thus 2.5x3600 coulombs x 3.6V = 32.4kJ = 9Wh

(Though here we can directly use hour as input and output use it:)

2.5Ah x 3.6V = 9Wh

So the cell will run 2.5A for an hour generating 9W

In general if you see very big or very small numbers, the prefix is wrong.

10000mAh is much better as 10Ah. Lots of marketing people think they can sell 10000 better
than 10, which is of course true where the victim is innumerate. Its harder to make a mistake
with numbers closer to 1.

This happens with big capacitors too, a 0.1F capacitor is often described as 100,000uF which is
as stupid as calling 10% "100000 parts per million"

The main advantage of SI units everywhere is there are no arbitrary constants to worry about,
1 watt is 1 volt amp, is 1 kg m/s^2, is 1 joule/second - 1 is good, you don't make mistakes with 1

One little wrinkle about SI is that the unit of mass is the kg, not the gramme. Its an historical
accident and its annoying, but its not going to be fixed in for foreseeable future.