Answer :
you determine which one to use by seeing which one would be easier
elimination: 5x + 7y = 2
-5x + 6y = 6
substitution: y = 9x + 5
2y = 7x + 2
elimination: 5x + 7y = 2
-5x + 6y = 6
substitution: y = 9x + 5
2y = 7x + 2