Show the following function is

**one to one**without invoking the horizontal line test.

f(x)= x^2-4x; x<2

P.S- obviously TW is not the place for math advice. Me and my friend have been trying to solve this for two hours. It is a bonus problem and our instructor said we cannot ask him how to do it as it is a bonus, otherwise that is what I would be doing.

Any help would be greatly appreciated!!