X = sillas
y = mesas
12x + 2y = 341
3x + 5y = 407
despejamos una de las dos ecuaciones
<img src="https://tex.z-dn.net/?f=%20y%20%3D%20%5Cfrac%7B341%20-%206x%7D%7B2%7D%20" />
<img src="https://tex.z-dn.net/?f=y%20%3D%20%5Cfrac%7B341%7D%7B2%7D%20-%206y" />
reemplazamos
<img src="https://tex.z-dn.net/?f=3x%20%2B%205%28%20%5Cfrac%7B341%7D%7B2%7D%20-%206x%29%20%3D%20407" />
<img src="https://tex.z-dn.net/?f=3x%20%2B%20%5Cfrac%7B1705%7D%7B2%7D%20-%2030x%20%3D%20407" />
<img src="https://tex.z-dn.net/?f=%20-%2027x%20%3D%20-%20%5Cfrac%7B891%7D%7B2%7D%20" />
<img src="https://tex.z-dn.net/?f=x%20%3D%2016.5" />
<img src="https://tex.z-dn.net/?f=y%20%3D%20%5Cfrac%7B341%7D%7B2%7D%20-%206%2816.5%29%20%3D%2071.5" />
<img src="https://tex.z-dn.net/?f=x%20%3D%2016.5%20%5C%5C%20y%20%3D%2071.5" />
espero haberte ayudado.