[tex]Ma=(a+b+c):3=21 \\ \\ Ma=a+b+c=21*3 \\ \\ \boxed{a+b+c=63} \\ \\ Ma=(a+b):2=25 \\ \\ Ma=a+b=25*2 \\ \\ \boxed{a+b=50} \\ \\ 50+c=63 \\ c=63-50 \\ \\ \boxed{c=13} \\ \\ 2a=c-7 \\ 2a=13-7 \\ 2a=6 \\ \\ \boxed{ a=3} \\ \\ 3+b=50 \\ b=50-3 \\ \\ \boxed{b=47}[/tex]