How do you use substitution for this problem

2x-y=4

x+3y=8

First:

2x-y=4

2x-4=y

y=2x-4

Then:

x+3 (2x-4) = 8

x+6x-12=8

7x=20

x=20/7

Then:

y=2x-4

y=2 (20/7) - 4

y=40/7-4

y=12/7

You need to rearrange the first equation to get y on its own:

2x-y=4

2x-4=y

y=2x-4

Then substitute this value for y into the second equation:

x+3 (2x-4) = 8

x+6x-12=8

7x=20

x=20/7

Then put this value into the equation you found for y:

y=2x-4

y=2 (20/7) - 4

y=40/7-4

y=12/7