Geometry (bisector) help please?

OC bisecs angle AOE

angle 1 = AOB

angle 2 = BOC

angle 3 = COD

angle 4 = DOE

measurement of angle 1 = 5a+2;

measurment of angle 2 = 2a-7;

Measurement of angle 3 = 4a;

measurement pf angle 4 = 7

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i worked it as

5a + 2 = 2a-7

added 7 to both sides and got

5a+9 = 2a

subtracted 2a from both sides and got

3a+9= 0

made it 3a+9 = 4a + 7

subtracted 3a from both sides and got

9 = a+7

subtracted 7 from both sides and got

2=a

which meant that

m or measurement of angle 1 = 12

m angle 2 = -3

m angle 3 = 8

m angle 4 = 7

angle 1 +2 should = 3+4 in my mind..but it isnt??

HELP!