You need to use the most important fact about angles in circles, which is:
- The angle subtended at the centre of a circle is twice the angle subtended at a point on the circumference by the same arc.
But you do need to understand what 'subtended' means.
In your diagram, look at the arc AE. The angle it subtends at the centre is angle AOE (the one we want in part (a)) and the angle it subtends at the circumference is angle ACE (which is 31 degrees). So angle AOE is twice angle ACE.
For part (b) use the fact that the angles on the straight line BOD add up to 180 degrees.
Hope you can do it now.
It is very easy if you can just multiply by two.