F3 maths

Provided that you have not mistyped the question (don't understand why there is 2PQs), the question can be solved using Pyth. theo.

draw the base and you can find that PQ can be moved leftward to form a right-angled triangle with SR as hypotenuse. Let the moved line be ST. TR = 9-6=3cm; SR = 5cm (given), then ST=4 cm. Hence, PQ = 4cm.

... But then it doesn't involve any 3-dimension stuff...