Answer
$14.14ms^{-1}$
Work Step by Step
The ball must come back with same speed as it was thrown with. lets compute the speed of ball with which it comes back to Nicole.
we have $ u= 10ms^{-1}$, $a=g =10ms^{-2} $ and $ s= 5.0m$
$\therefore 2as =v^2 -u^2$
$ 2*10*5 = V^2 - 10^2$
$ v^2 = 200$
$ v= 14.14ms^{-1}$
hence speed of ball with which Nicole has thrown it up is $14.14 ms^{-1}$
