With center O draw small circle with radius 2cm and big circle with radius 5cm.

Take point P on big circle and connect point P and Point A.

Bisect line PA and at the bisect point take point M.

Take distance MO and draw circle with radius MO.

At the matching point of OM radius circle with small circle of 2cm take point A and B.

Now connect point P and Point A and Point P and Point B those two lines are tangent to the small circle.

Now measure distance of line PA that is 4.5 Approximately.

PA = 4.5 (Approximately)