Hello

1) AO is an axis of symmetry of the figure => angle BOA=angle COA

OC=OA => triangle COA is isosceles => angle OCA=angle OAC

Sum of angles of triangle COA=180°

angle COA + angle OCA + angle OAC = 180°

angle COA + 2 x angle OAC = 180° (1)

PQ is tangent to the circle => angle OAQ = 90°

But angle OAQ = angle OAC + angle CAQ = 90°

Therefore angle OAC = 90° - angle CAQ (2)

(1) and (2)

angle COA + 2 x (90° - angle CAQ) = 180°

angle COA - 2 x angle CAQ = 0