Since both 4x-9 and x-3 equal Y, you can equate them to each other. This is the transitive property (a = b and b = c, then a = c).
Another way to think of this is that you are substituting 4x-9 in for Y in the equation x-3.
Either way, the equation is
4x - 9 = x - 3
3x = 6
x = 2
Substituting into either of the original equations to find y,
Y = 4x - 9
Y = 4(2) - 9
Y = -1