It seems like your question is incomplete or unclear. If "xmxm" refers to a specific concept, term, or abbreviation, please provide more context so I can give you a precise answer.
P,xmxm
Cmxcix + dclxvi = mdclxvImproved answer in detail:-999 in Roman numerals: CM+XC+IX = IM666 in Roman numerals: DC+LX+VI = DCLXVI999+666 = IM+DCLXVI = MDCLXVNote that the Romans would have probably total up CM+XC+IX in the following manner:-CM+XC = XMXM+IX = IMMAnd that they would have probably total up DC+LX+VI in the following manner:-DC+LX = DCLXDCLX+VI = DCLXVITherefore it follows that:- IM+DCLXVI = MDCLXV (1665)Roman numerals:- M+1000, D=500, C=100, L=50, X=10, V=5 and I=1