[tex] \left \{ {{x+y=18\:(I)} \atop {2x+3y=40\:(II)}} \right. [/tex]
simplify by (-2) first equation
[tex] \left \{ {{x+y=18\:(-2)} \atop {2x+3y=40\:\:\:\:}} \right. [/tex]
[tex] \left \{ {{-\diagup\!\!\!\!\!2x-2y=-36} \atop {\diagup\!\!\!\!\!2x+3y=40}} \right. [/tex]
[tex] \left \{ {{-2y=-36} \atop {3y=40}} \right. [/tex]
[tex]\boxed{\boxed{y = 4}} \end{array}}\qquad\quad\checkmark[/tex]
Replace the found value of "y" in the first equation.
[tex]x+y=18\:(I)[/tex]
[tex]x + 4 = 18[/tex]
[tex]x = 18 - 4[/tex]
[tex]\boxed{\boxed{x = 14}} \end{array}}\qquad\quad\checkmark[/tex]
