So you're required to show that a+b=c
Here are some hints:
The angle at the centre is twice the angle at the circumference subtended by the same arc.
The sum of the measures of the interior angles in any convex quadrilateral is 360 degrees.
Try to find a quadrilateral with angles expressed using a,b,c or a scaled version of them.
It's a 2 or 3 line solution.