Answer
$157~kg~~$ of gas must be ejected each second.
Work Step by Step
We can use Newton's Second Law to find the required thrust:
$\sum F = ma$
$F_{thrust}-mg = ma$
$F_{thrust} = m~(a+g)$
$F_{thrust} = (6100~kg)~(21~m/s^2+9.8~m/s^2)$
$F_{thrust} = 187,880~N$
We can find $R$, the rate at which the gas must be ejected:
$R~v_{rel} = F_{thrust}$
$R = \frac{F_{thrust}}{v_{rel}}$
$R = \frac{187,880~N}{1200~m/s}$
$R = 157~kg/s$
$157~kg~~$ of gas must be ejected each second.
