a(n) = 3*binomial(n+1,6).

Terms

a(0) =3a(1) =21a(2) =84a(3) =252a(4) =630a(5) =1386a(6) =2772a(7) =5148a(8) =9009a(9) =15015a(10) =24024a(11) =37128a(12) =55692a(13) =81396a(14) =116280a(15) =162792a(16) =223839a(17) =302841a(18) =403788a(19) =531300a(20) =690690a(21) =888030a(22) =1130220a(23) =1425060a(24) =1781325a(25) =2208843a(26) =2718576a(27) =3322704a(28) =4034712a(29) =4869480