How to do it...

1. First, we need to add the GetLandingTime function to compute the landing time:

public float GetLandingTime (float height = 0.0f) 
{ 
Vector3 position = transform.position; 
float time = 0.0f; 
float valueInt = (direction.y * direction.y) * (speed * speed); 
valueInt = valueInt - (Physics.gravity.y * 2 * (position.y - height)); 
valueInt = Mathf.Sqrt(valueInt); 
float valueAdd = (-direction.y) * speed; 
float valueSub = (-direction.y) * speed; 
valueAdd = (valueAdd + valueInt) / Physics.gravity.y; 
valueSub = (valueSub - valueInt) / Physics.gravity.y; 
if (float.IsNaN(valueAdd) && !float.IsNaN(valueSub)) 
return valueSub; 
else if (!float.IsNaN(valueAdd) && float.IsNaN(valueSub)) 
return valueAdd; 
else if (float.IsNaN(valueAdd) && float.IsNaN(valueSub)) 
return -1.0f; 
time = Mathf.Max(valueAdd, valueSub); 
return time; 
} 

2. Now, we add the GetLandingPos function to predict the landing spot:

public Vector3 GetLandingPos (float height = 0.0f) 
{ 
Vector3 landingPos = Vector3.zero; 
float time = GetLandingTime(); 
if (time < 0.0f) 
return landingPos; 
landingPos.y = height; 
landingPos.x = firePos.x + direction.x * speed * time; 
landingPos.z = firePos.z + direction.z * speed * time; 
return landingPos; 
}