Let's focus this statement:-
if it is given the maximum requirement for combination of P1 and P3 is 9
it means that at any time sum of maximum resources taken by P1& P3 is 9
now lets see all possiblties:-
P1:P3 :P2 = (4,5)(3,6)(2,7)(1,8)
now for following cases consider the condition for no deadlock:- "allocate n-1 resource to evry process after then allocating one more resource guarantee of no_deadlock."
(4,5,6) =(4-1,5-1,6-1)+1=13
(3,6,6)=(2,5,5)+1=13
(2,7,6)=(1,6,5)+1=13
(1,8,6)=(0,7,5)+1=13