If we know two angles we do in fact know all three, exactly, because the three angles add up to 180°, so finding the third angle is always the first thing we do.

C = 180° - 36° - 31° = 113°

Your calculator will give a and c to ten significant figures but you will round them to the nearest millimetre when you write them down.

So C = 113° a = 87 mm and c = 136 mm