And the coordinates of vector P in cardinal numbers A, B and C are (1, 2,3), so vector P=a+2b+3c. So there are:
X+y= 1, x-y=2, z=3, the solution is: X = 3/2, Y =- 1/2, Z = 3. Therefore, the coordinates of vector P under bases a+b, a-b and C are (3/2,-).