x+9y=8 (1)

x=3-9y (2)

x (3-9y)+9=8
3x -9y +9y=8
3x = 8
x= 2.6
Is this correct?

If you are using "substitution", your third line makes no sense

x + 9y = 8
3-9y + 9y = 8
which results in the statement
3 = 8 , which of course is silly

Your system of equations has no solutions.