Answer
$a=22.37\frac{m}{s^2}$
Work Step by Step
First we calculate the volume of the balloon:
$V=\frac{4\pi r^3}{3}=\frac{4\pi (12m/2)^3}{3}=904.78m^3$
Then the bouyancy force is:
$F_{b}=\rho_{air}gV=1.16\frac{kg}{m^3}*9.81\frac{m}{s^2}*904.78m^3=10296.03N$
On the other hand the total mass will be the sum of the helium mass and people mass:
$m_{He}=\rho_{He}*V_{He}=\frac{1.16}{7}\frac{kg}{m^3}*904.78m^3=149.93kg$
$m_{people}=2*85kg=170kg$
$m_{total}=149.93kg+170kg=319.93kg$
And the total weight is:
$W=m_{total}*g=319.93kg*9.81\frac{m}{s^2}=3138.51N$
So the net force will be:
$F_{net}=F_{b}-W=10296.03N-3138.51N=7157.52N$
Finally the acceleration will be:
$a=\frac{F_{net}}{m_{total}}=\frac{7157.52N}{319.93kg}=22.37\frac{m}{s^2}$
