Compton scattering, formulae question

[channel1]

Homework Statement

An X-ray source of wavelength gamma is directed at a sample; determine the maximum speed of scattered electrons.

Homework Equations

KE=($\gamma$-1)mc²=hc/$\lambda$ - hc/$\lambda$^'

The Attempt at a Solution

I've been provided with the solution using the formula above, however it looks like it's E_incident - E_scattered ---or in other words initial minus final...isn't convention final-initial? Why do we subtract initial minus final in this case?

 

[gildomar]

The final is subtracted because the scattered photon will have a lower energy than the incident photon (since some of it has gone to the electron).

 

[vela]

Apply conservation of energy to the situation.

This question really doesn't belong in the advanced physics section.