A boat covers a distance of 36 km downstream in 3 hours and the same distance upstream in 6 hours. Let vb km/h denote the speed of the boat in still water and vc km/h denote the speed of the current.
(i) Form two linear equations in vb and vc based on the given information and express them in the matrix form AX=Y.
(ii) Assuming the coefficient matrix is invertible, find the values of vb and vc.