Hi,
from the sum of l-m+2n,3l-2m+n and 4l+5m-6n
subtract the sum of l +m +n and 2l+3m +4n​
(l-m+2n + 3l-2m+n + 4l+5m-6n) - ( l + m + n + 2l+3m +4n​) =
73 +2m -3n -(22 + 4m + 5n)
73 +2m -3n -22 - 4m - 5n
51 - 2m - 8n
Wish You the Best in your Studies.