x-y=2k ①
x+3y=1-5k ②
①+②得2x+2y=1-3k
x+y=1/2-3/2k ③
①+③得2x=1/2+1/2k
x=1/4+1/4k
③-①得2y=1/2-7/2k
x+y+x-5y