C. The first is a solution, but the second is not.
5x - y/3 = 13 ; (2,-9)
substitute the letters:
5(2) - (-9/3) = 13
10 - (-3) = 13 : note that deducting a number with a negative sign turns both sign as positive.
10 + 3 = 13 ;
13 = 13
5x - y/3 = 13 ; (3,-6)
5(3) - (-6/3) = 13
15 - (-2) = 13
15 + 2 = 13
17 = 13 not equal. not a solution
hope i could help
