When functions are defined by more then one equation, they are called piecewise defined functions
The sum of the opposite interior angles is equal to the exterior angle.
So,
∠CAB + ∠ACB = ∠CBD
x + 40 + 3x + 10 = 6x
4x + 50 = 6x
2x = 50
x = 25°
∠CAB = x + 40 = 25 + 40 = 65°
Hence, the answer is D.
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