Let numbers be a and b
Adding both
[tex]\\ \qquad\quad\sf\longmapsto 2a=24[/tex]
[tex]\\ \qquad\quad\sf\longmapsto a=\dfrac{24}{2}[/tex]
[tex]\\ \qquad\quad\sf\longmapsto a=12[/tex]
[tex]\\ \qquad\quad\sf\longmapsto 12-b=9[/tex]
[tex]\\ \qquad\quad\sf\longmapsto b=12-9[/tex]
[tex]\\ \qquad\quad\sf\longmapsto b=3[/tex]