Answer:
The value is 

Step-by-step explanation:
From the question we are told that 
 The total time of flight is 
 
Generally from kinematic equation 
 

So v is the velocity at maximum height and the value is v = 0 m/s 
So 
 

=> 

Here u is the initial velocity of the dart as it leaves that gun 
Gnerally the horizontal range of the dart is mathematically represented as 
 

For maximum horizontal range the value of 
 
So 
 

=>  
