m = 0.6 (slider from -20 to 20, step 0.0010)
px = -2 (slider from -5 to 5, step 0.1)
py = 0 (slider from -5 to 5, step 0.1)
a: y = m * (x - px) + py
A: (px, py)
B, C = Intersect(c, a)