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