Which expression can you substitute for xin12x+y=5 to solve the system below? A:6-y
B:5-y
C:5+y
D:5-12y

Well, you can see in the system that they give you the expression x = 6 - y.  So it's literally telling you that, well, x equals (6 - y), and therefore since they're equivalent expressions, you can substitute 6 - y for x to solve the system.

if x = 1, then the answer is 3.5