You don't even need to know the measure of arc PM. All you need to notice is that inscribed angle MPN and inscribed angle MON subtend the same arc MN. Since the measure of an inscribed angle is 1/2 of its subtended arc, both angles MPN and MON have measure of 1/2 of the measure or arc MN which they subtend. So they have equal measure. Therefore the measure of angle MON is also 42°. It didn't require any calculations at all.Edwin