Question 37062
8x - 4y = 16
y = 2x - 4

In this system you have the y isolated in the second equation so you only have to substitute it in the first.

8x-4(2x-4)=16
8x-8x+16=16
8x-8x=16-16
0=0 

This is not a system of equation this is an identity, we have obtain an identity because the first and the second equations are the same but with different coefficients.