Sir as per which theorem it is red encircled part is comming pls tell

see , (1+3k)/10 gives k = 0 and 3 . in case 1 , if k = 0 then we have 1+ 3×0= 1 subgroup , and case 2 , if k =3 then we have 1+3×3 = 10 subgroup..

put k = 3 in (1+3k) , we get 10