Solve equation [2] for the variable x
[2] x = y + 6
Plug this in for variable x in equation [1]
[1] (y +6) + y = 20
[1] 2y = 14
Solve equation [1] for the variable y
[1] 2y = 14
[1] y = 7
By now we know this much :
x = y+6
y = 7
Use the y value to solve for x
x = (7)+6 = 13