a(n) = 2n+7^n-5^n.

Terms

a(0) =0a(1) =4a(2) =28a(3) =224a(4) =1784a(5) =13692a(6) =102036a(7) =745432a(8) =5374192a(9) =38400500a(10) =272709644a(11) =1928498640a(12) =13597146600a(13) =95668307308a(14) =672119557252