Answer
α = $ 30^{\circ}$
Work Step by Step
In given figure 21-
$\angle$ CDB is right angle and ADB is a straight line, therefore $\angle$ CDA is also a right angle and hence triangle CDA is a right triangle.
In right triangle CDA, C and A are acute angles, therefore-
C + A = $90^{\circ}$
α + 2 α = $90^{\circ}$ ( Substituting values of C and A )
3 α = $90^{\circ}$
α = $\frac{90^{\circ}}{3}$ = $30^{\circ}$
