Help me please

What does it call to be solved for?

I am solving for x and y

nx+my = a

-nx+my = b        add the two equations  and we get

2my  =  a + b        divide both sides by 2m

y  =  (a + b) / (2m)

Put this back into the first equation for y

nx  +  m [ (a + b) / (2m)]  = a      simplify

nx +  (a + b) / 2    = a           subtract (a + b) / 2   from both sides

nx  =  a  -  (a + b) / 2         we can write this as

nx  =   [ 2a  - a - b ]  / 2

nx =  [ a - b] / 2       divide both sides by n

x =  ( a - b ) / (2n)