Maths problem: how to do, thanks.?

CD/AC = tan(∠CAD) = tan(32°) = a = 0.6248693519
BC/AC = tan(∠CAB) = tan(32°+∠DAB) = 2a
Let θ = ∠DAB, x = tan(θ)
2a = (a + x)/(1 - ax)
∴ a + x = 2a - 2a^2 x
∴ x = a/(1+2a^2) = 0.3508681772 = tan(θ)
∴ θ = 19°
∴ ∠CAB = 32°+19° = 51°
∴ ∠B = 39°
B 選項正確.