[tex]\left\{\begin{array}{ccc}y=x+3&(1)\\4x+y=18&(2)\end{array}\right\\\\\text{substitute (1) to (2):}\\\\4x+(x+3)=18\\\\(4x+x)+3=18\qquad\text{subtract 3 from both sides}\\\\5x=15\qquad\text{divide both sides by 5}\\\\x=3\\\\\text{put the value of x to (1):}\\\\y=3+3\\\\y=6[/tex]
