a

a = 2 (slider from -10 to 10, step 0.1)
b = 5.2 (slider from -10 to 10, step 0.1)
f(x) = a * x + b
A = Intersect(f, xAxis)
B: (x(A) + 1.0, 0.0)
C = Point(yAxis)
c = Line(C, xAxis)
D = Intersect(f, c)
α = Angle(B, A, D)