[tex]\it a = 3^{89}\cdot2 = 3^{88}\cdot3\cdot2 = 3^{88}\cdot6
\\\;\\
b = 3^{88}
\\\;\\
6\cdot3^{87} = 2\cdot3\cdot3^{87} =2\cdot 3^{88}[/tex]
[tex]\it m_a = \dfrac{a+b+c}{3} =\dfrac{6\cdot3^{88}+3^{88}+2\cdot3^{88}}{3} =\dfrac{3^{88}(6+1+2)}{3} =
\\\;\\ \\\;\\
=\dfrac{3^{88}\cdot9}{3} =3^{88}\cdot3 = 3^{89}[/tex]