System By Substitution

Y = 4x-9

Y=x-3

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