0, 6, 23, 56, 109, 184, 279, 384, 473, 488, ___, ___, . . .

Let n stand for the position in the sequence (n=1,2,3,4...) and let d[n] be the value of the nth number in the sequence.  Establish the initial value of variable a for n=0:

a[0]=5;

The terms in the sequence can be found for n¡Ý1 by the equation

d[n]=d[n-1]+c[n]

where the relationship between variables a,b,c and d are:

c[n]=c[n-1]+b[n];

b[n]=b[n-1]+a[n];

a[n]=a[n-1]-(2^n-1).

The solutions for n=11 and n=12 are:

307, -320

I hope this makes sense.  The 13th number in the sequence would be -1899.