Bullet Drop Calc

// calculated bullet drop
                            targetDrop.X = (float) (Math.Pow(aimTarget.Distance, 2) - Math.Pow(AIM3D.Y, 2));
                            targetDrop.Y = AIM3D.Y - Main.Player.Position.Y;
                            dropAngle = (float)Math.Atan((Math.Pow(startSpeed, 2) - Math.Sqrt(Math.Pow(startSpeed, 4) - 9.81 * (9.81 * Math.Pow(targetDrop.X, 2) + 2 * targetDrop.Y * Math.Pow(startSpeed, 2)))) / (9.81 * targetDrop.X));

                            A1 = (float)Math.Tan(dropAngle) - 0;
                            B1 = 0-1;
                            C1 = A1 * 0 + B1 * 0;

                            A2 = (targetDrop.Y + 1) - targetDrop.Y;
                            B2 = targetDrop.X - targetDrop.X;
                            C2 = A1 * 0 + B1 * targetDrop.Y;

                            float det = A1*B2 - A2*B1;
                            if(det == 0)
                            {
                                //Lines are parallel
                            }
                            else
                            {
                                lineInterX = (B2 * C1 - B1 * C2) / det;
                                lineInterY = (A1 * C2 - A2 * C1) / det;
                            }
                            AIM3D.Y += lineInterX - targetDrop.Y;
                            AIM = Graphics.W2S(AIM3D); // project to screen