Linear Function: y = mx + b (2 points given) in code

In your posted code, you seem to have made a typo. This:

var m = (point2.Y - point1.Y) / (point2.X + point1.Y);

...should be:

var m = (point2.Y - point1.Y) / (point2.X - point1.X);

You want this:

public static float GetY(Vector2 point1, Vector2 point2, float x)
    {
        var dx = point2.X - point1.x;  //This part has problem in your code
        if (dx == 0)
            return float.NaN;
        var m = (point2.Y - point1.Y) / dx;
        var b = point1.Y - (m * point1.X);

        return m*x + b;
    }

I would have thought that:

var m = (point2.Y - point1.Y) / (point2.X + point1.Y);

should be

var m = (point2.Y - point1.Y) / (point2.X - point1.X);

Gradient is the delta in Y divided by the delta in X.