Muzzle Speed of Gun with 24.5 km Range

[P-Illiterate]

Homework Statement

Find the muzzle speed of a gun whose maximum range is 24.5 km.

Homework Equations

R = V₀²sin2angle/g

The Attempt at a Solution

How do I find the angle?

Also can you check what I did wrong in this one?
A projectile is fired at a speed of 840 /sec at an angle of 60 degree. How long will it take to get 21 km downrange?
change in X = V₁t
21,000/840 = t
t=25 sec
But the right answer is 50 s

 

[NobodySpecial]

The angle for maximum range should be obvious - think what happens if you shoot straight up or straight horizontal?