A projectile fired at \( 30^\circ \) to the ground is observed to be at same height at time \( 3 \, \text{s} \) and \( 5 \, \text{s} \) after projection, during its flight. The speed of projection of the projectile is _________ \( \text{m s}^{-1} \).
(Given \( g = 10 \, \text{m s}^{-2} \))
