Need of help - applying calculus to the physical world

i urgently need help with this question :(.

- To lift a weight a boy uses a rope 20 metres long thrown over a tree branch which is 6 metres off the ground. He ties the rope around his waist (1 metre off the ground) and walks off at a constant speed of 1ms^(-1). At time, *t*, let the boy be x metres from the vertical below the weight and let the weight be y metres above the ground.

Find how fast the weight is rising when it is 3 metres above the ground.