Firstly, determine the radius OA of the circle.
Note; given a coordinate on the circumference of a circle, where the radius and the tangent are both incident on that coordinate, there will exist an angle of 90° between the the 2 straight lines.
Applyling trigonometry;
Sin 30° = OA ÷ OB thus OA = 0.5 x OB which is OA = 0.5 x 20
Therefore Radius OA = 10
Now construct a right angled triangle (RAT) where OP is the Hypotenuse, the coordinate on the x axis is 3p hence the length of the adjacent side to angle POB. The coordinate 2p will form the opposite side to angle POB and also acts as the other length of the RAT.
Now apply Pythagoras Theorem;
OP² = (3p)² + (2p)² -----> 100 = 9p² + 4p²
thus p² = 100 ÷ 13 -------> p = √7.7 ---->
Answer p = 2.8 to 1 d.p