You do have a number, it's just not written down! It's that a full circle is 360 degrees

• Let x + y = d and w + z = e for easier writing
• We know d + e = 360 degrees (a full circle)
• d = e, so 2d = 360, so d = 180 degrees
• The angle AOB = d, and d = 180 degrees, so AB is a straight line

The actual maths is really easy, the bit that I assume is tripping you up is the realisation that you have more information than the words of the question