$u=0, a=2 \,ms ^{-2}$

The velocity of object after one second

$v=u+a t$

$\Rightarrow v=2 \,ms ^{-1}$

$s=\frac{1}{2} \times 2 \times 1^2=1 \,m$

Now after separating from the balloon it will move under the effect of gravity alone.

$-h=v t-\frac{1}{2} \times 9.8 \times t^2$

$\Rightarrow-1=2 t-4.9 t^2$

$\Rightarrow 4.9 t^2-2 t-1=0$

$\Rightarrow t=0.7 \,s$