Motion In A Plane

A projectile fired at \(30^{\circ}\) to the ground is observed to be at same height at time \(3 \mathrm{~s}\) and \(5 \mathrm{~s}\) after projection, during its flight. The speed of projection of the projectile is ___________ \(\mathrm{m} ~\mathrm{s}^{-1}\).

(Given \(g=10 \mathrm{~ms}^{-2}\) )