The route an aircraft takes is as follows: Firstly, along a route from O to A described by the vector 11a +2b. Next, along a route from A to B described by the vector 10a +3b. Thirdly, along a route from B to C described by the vector -17a – 7b and finally, along a route from C to D described by the vector 8a + 3b. Find the route that the plane will now have to take to return from D to O. The return journey is along ca + yb. What are the numbers a and y?