Hi,I have a question that I have problems with.It's my homework due Thursday.

M1(9,-3);M2(-6,5).Bring the origin to M2 and rotate the system so that M1,M2 agrees with the direction of the new x axis.(tanθ=-8/15).

I can't understand what I'm supposed to find as a solution.Why do I need to change the origin?Should I only show how I did it?Thanks in advance.(I'm not asking for having my hw done by sb else.I studied all weekend on this subject but I can't understand the question.)