Answer
$a_g=4.2\frac{m}{s^2}$
Work Step by Step
For $R$, the mass is $M$ and for $2R$, the mass is $4M$. So for $3R$, the total mass is $M+4M=5M$
Using the formula below,
$a_g=\frac{GM}{R^2}$
$a_g=\frac{G(5M)}{(3R)^2}$
$a_g=\frac{6.67\times 10^{-11}\times 5\times 4.1\times 10^{24}}{(3\times 6\times10^6)^2}$
$a_g=4.2\frac{m}{s^2}$
