profit was maximised where the materials and labour constraints crossed, which gave 4000 units of x (tennis rackets), and 6000 units of y (badminton rackets), which is indeed more then the contract, which is 1000 units of x and 2000 units of of y. However, I have just noticed that at the start of the question it specified that there is demand for 6000 x and 6000 y, although my profit maximisation only gives 4000x and 6000y. Is this "part D" something to do with buying enough materials so that those extra 2000 units of x are sold, so that demand for x is met at 6000 units ? (demand for y however seemed to be already met with the current maximisation).

Furthermore, the shadow price for materials was calculated as £66 (part C), although I don't know if this has anything to do with helping me solve part D?

Still not sure where to start though, if materials are now available at £15 per kg, this would change the materials constraint to 0.6x + 0.4y <=4000. Should I just solve the whole thing again using this constraint?