When you solve for y= 5- 2x, you are just saying that "y" and "5- 2x"arethe same thing. Whatever you do to one, you can do to the other. The first equation says that if you multiply y by 3 and add 4x, you get -8. Since y is the same as 5-2x, if you multiply 5-2x by 3 and add 4x, you still get -8: 4x+ 3y= 4x+ 3(5- 2x)= 4x+ 15- 6x= 15- 2x= -8