[tex]\displaystyle \left \{ {{x+y=4} \atop {3x+4y=13}} \right.\\ \\\left \{ {{x=4-y} \atop {3x+4y=13}} \right. \\\\3(4-y)+4y=13\\y=1\\x=4-1\\x=3\\(x,y)=(3,1)[/tex]