[tex]\left\{\begin{array}{ccc}3x+y=9\\y=3x+6&\text{subtract 3x from both sides}\end{array}\right\\\underline{+\left\{\begin{array}{ccc}3x+y=9\\-3x+y=6\end{array}\right}\qquad\text{add both sides of the equations}\\.\qquad\qquad2y=15\qquad\text{divide both sides by 2}\\.\qquad\qquad\boxed{y=7.5}\\\\\text{Put the value of y to the first equation}\\\\3x+7.5=9\qquad\text{subtract 7.5 from both sides}\\3x=1.5\qquad\text{divide both sides by 3}\\\boxed{x=0.5}\\\\Answer:\ \boxed{x=0.5\ and\ y=7.5\to(0.5,\ 7.5)}[/tex]
