Determining required magnitude for trajectory

The following method can be used to determine the required initial magnitude required for a fired object to pass through a point at RelativeX, RelativeY.

private float CalculateRequiredStartingMagnitude(float relativeX, float relativeY, float desiredAngle, float gravity)
{
    double topOfInsideOfSquareRoot = 2 * (relativeY * Math.Tan(desiredAngle) - relativeY);
    double squareRootPart = Math.Sqrt(topOfInsideOfSquareRoot / Math.Abs(gravity));
    double xVelocity = relativeY / squareRootPart;

    return (float)xVelocity / (float)Math.Cos(desiredAngle);
}