Please help. I don't understand

∠ACB = 72

Step-by-step explanation:

OAD is straight line.

∠DAB +  ∠OAB  = 180

142 + ∠OAB  = 180

          ∠OAB = 180 - 142

          ∠OAB =  38

ΔAOB is isosceles triangle. OA = OB = radius

∠OAB = ∠OBA = 38   {angles opposite to equal sides or equal}

In ΔAOB ,

∠OAB + ∠OBA + ∠AOB = 180   {Anlge sum property of triangle}

   38 + 38 + ∠AOB = 180

               76 + ∠AOB = 180

                       ∠AOB = 180 - 76

                       ∠AOB = 104

THe angle subtended by an arc of a circle at the center is double the angle subtended by it at any point on the remaining part of the circle

∠AOB =  2*∠ACB

∠ACB = ∠AOB/2

         = 104/2

∠ACB = 72

341

step-by-step explanation:

all you have to do is 9x31 (which equals 279) and then 20x31 (which equals 620). then you have to subtract. 620-279=341.