\(10 \,cm \Rightarrow 10 \times 10^{-2} \,m \Rightarrow 10^{-1} \Rightarrow 0.1 \,m\)

It is a case of horizontal projectile.

So, \(a_x=0, u_x=4, u_y=0, a_y=-g\)

\(R=100 \,m , T=\sqrt{\frac{2 H}{g}} \Rightarrow\) Time of flight

\(R=u_x T\)

\(100=u \sqrt{\frac{2 \times 0.1}{100}} \Rightarrow \frac{u \sqrt{2}}{10}=100\)

\(u=\frac{1000}{\sqrt{2}} \approx 707 \,ms ^{-1}\)