Hello there! :)
It's a pleasure to be helping you today with your physics question!
Answer:

Step-by-step explanation:
We want to find the initial speed of the ball.
To do this, we have to apply the formula for the time of flight of a projectile:

where θ = angle of flight
g = acceleration due to gravity
v0 = initial speed
Therefore, substituting the given values into the formula, we have that:

⇒ 2 ×
×0.8910= 9.8 × 4.2
⇒


That is the initial speed of the ball.
I hope this helps you!
Good Luck with your Assignment!