How can you decide whether to add or subtract to eliminate a variable in a linear system? can you explain it plz : )

To eliminate variables you are technically supposed to add but with the correct circumstances you can obviously subtract too. You want them to cancel out and equal 0. For example:
1+(-1)=0,
x+(-x)=0,
6y+(-6y)=0.
Make sure you have a positive and a negative variable and that they have the same coefficient so they will cancel out:)