Hi there!
We can use substitution to solve this equation.
3x + y = 18
y = 3x
Substitute y from second equation to first one.
3x + 3x = 18
6x = 18
x = 3
Now substitute x = 3 to any of the equation to find y.
3x + y = 18
3(3) + y = 18
9 + y = 18
y = 9
So your solution is ( 3 , 9 ).
Hope this helped! :)