Ответ:

b = 7

Пошаговое объяснение:

[tex]\displaystyle \left \{ {{6a+b=7} \atop {a-b=0}} \right. \left \{ {{6a+b=7} \atop {a=b\hfill }} \right. \left \{ {{6b+b=7} \atop {a=b\hfill}} \right. \left \{ {{7b=7} \atop {a=b}} \right. \left \{ {{b=7} \atop {a=7}} \right.[/tex]