alternate is statement

i have attached a copy of an excel file and if you look at the end i am trying to write an IF statement that reads if J = "2-0" then column M = 12-(K) or 12--4 so answer is 16 and then column N = -column M or -16 and then if J = "2-1" then M = 6-(K) or 6--0 = 6 and then N = -M .....

so if J was "2-1" and (K) -4 then the answer given for column M would be 6--4=10 and column N = -10

Much appreciated
Troy