The correct answer is q + r = p.
The reason that we know this is that we know all the interior angles will add up to 180. We also know the angle opposite to p (let's call it s) will be equal to 180 - p since together they create a straight line. So, we can set up this equation and solve using that substitutation.
q + r + s = 180 ---> Sub in p value
q + r + (180 - p) = 180 ----> Subtract 180 from both sides
q + r - p = 0 ----> Add p to both sides
q + r = p
