If a ball thrown vertically upward falls to the ground after 3 s, then the speed of the ball at the moment of falling is …

The origin of the coordinate system is at the point where the ball is thrown. The Y-axis is directed upwards.
The Y coordinate became zero after 3 s.
Expression for Y coordinate:
Y = V₀t – gt² / 2.
The same expression for t = 3 s:
0 = V₀ * 3 s – (10 m / s² * 3² c²) / 2;
V₀ * 3 c = 45 m;
V₀ = 15 m / s.
The speed at the moment of falling is:
V = V₀ – gt = 15 m / s – 10 m / s² * 3 s = -15 m / s (minus sign, because the velocity vector is directed downward).
The absolute value of V is 15 m.
Answer: 15 m / s.