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; 
}