Answer:
x = 60°
Step-by-step explanation:
From the figure and the fact that AB and CD are straight lines we can see that:
30° + 2x + ∠DOB = 180° (eq. 1)
x + ∠DOB = 90° (eq. 2)
Subtracting equation 2 to equation 1 we get:
30° + 2x + ∠DOB - (x + ∠DOB) = 180° - 90°
30° + x = 90°
x = 90° - 30°
x = 60°
