X=y+6
x+6y=20
solving systems of equations using substitution

To get the answer we can subsittute first equation into second.
x+6y=20
y+6+6y=20
6+7y=20        /-6
7y=14        /:2
y=2
Now we can back substitute
x=y+6
x=2+6
x=8
Finally we get
x=8
y=2

Looks pretty good to me!