a(n) = 7*(n+1)*binomial(n+2,7)/2.

Terms

a(0) =21a(1) =196a(2) =1008a(3) =3780a(4) =11550a(5) =30492a(6) =72072a(7) =156156a(8) =315315a(9) =600600a(10) =1089088a(11) =1893528a(12) =3174444a(13) =5155080a(14) =8139600a(15) =12534984a(16) =18877089a(17) =27861372a(18) =40378800a(19) =57557500a(20) =80810730a(21) =111891780a(22) =152956440a(23) =206633700a(24) =276105375a(25) =365195376a(26) =478469376a(27) =621345648a(28) =800217880