39.
Consider the following for the next two (02) items that follow:
The top (M) of a tower is observed from three points P, Q and R lying in a horizontal straight line which passes directly along the foot (N) of the tower. The angles of elevations of M from P, Q and R are \( 30^{\circ} \), \( 45^{\circ} \) and \( 60^{\circ} \) respectively. Let PQ = \( a \) and QR = \( b \).
What is PN equal to?