I'm learning calculus on my own pretty much, and even made a sample app on finding the easiest way to find a derivative of a polynomial. My program graphs the polynomial (which is a parabola), and the derivative (which is a straight line representing slope). When the line intersects the parabola, I have it draw green end points. But I want to be able to only draw the line within the 2 end points. Any ideas on how to do this? Here's my code:

Code:
Option Explicit

Private Type Point_2D
    
    X As Single
    Y As Single

End Type

Private Center As Point_2D

Private X As Single, Y As Single

Private X2 As Single, Y2 As Single

Private Current_X As Single

Private Initial As Point_2D

Private Pos As Point_2D, Pos2 As Point_2D

Private Scalar As Point_2D

Private Sub Form_Activate()

    'Easiest way to find the derivative of a polynomial:
    '---------------------------------------------------
    'A derivative is the slope of a tangent line
    '
    'Formula for finding a derivative of X^n: n(x^(n - 1))
    '
    'Example:
    '
    'd/dx(x^2)
    '= 2(X^(2-1))
    '= 2X^1
    '= 2X
    
    'So the derivative of X^2 is 2X.

    ScaleMode = 3
    AutoRedraw = True
    BackColor = RGB(0, 0, 0)
    
    Center.X = ScaleWidth / 2
    Center.Y = ScaleHeight / 2
    
    Initial.X = Center.X
    Initial.Y = Center.Y
    
    Scalar.X = 25
    Scalar.Y = 5
    
    For Current_X = -100 To 100 Step 0.01

        X = Current_X
        Y = X ^ 2
        
        Pos.X = Initial.X + X * Scalar.X
        Pos.Y = Initial.Y - Y * Scalar.Y
        
        PSet (Pos.X, Pos.Y), RGB(255, 0, 0)
        
        '------------------------------------
        X2 = Current_X
        Y2 = 2 * X2
        
        Pos2.X = Initial.X + X2 * Scalar.X
        Pos2.Y = Initial.Y - Y2 * Scalar.Y
        
        If Int(Pos.X) = Int(Pos2.X) And Int(Pos.Y) = Int(Pos2.Y) Then
        
            'Draw end points for secant line.
        
            PSet (Pos2.X, Pos2.Y), RGB(0, 255, 0)
            
        Else
        
            PSet (Pos2.X, Pos2.Y), RGB(255, 0, 0)
            
        End If
        
    Next Current_X

End Sub