2x + y = 10

y = - 2x + 8

you cant solve.

By using substitution, this equation will be easy to solve. Plug the second equation into the first equation and solve like you normally would to find x, then use that x value to plug it into y.

2x + (-2x+8) = 10

8=10

In this case, the variable was cancelled out itself, which means that this problem is undefined.