How do i do substitution for this problem
y=4x-9
y=x-3

4x-9=x-3

4x-x=-3+9

3x=6

x=2

y=2-3

y=-1

(x, y) = (2, -1)

Well you know that y=4x-9 for first one. So if that is what y equals then that means that you could replace y with 4x-9. If you did it I. first equation it would be:
4x-9 = 4x-9

That is pointless. But if you do it in second equation you get:
4x-9 = x-3
3x = 6
x=2

since x = 2 we can do:
y = x - 3
y = 2 - 3
y = -1

So solution is (2, -1)