In Exercise T, we are looking for number of triangles on `3\times n` grid.
This problem is about quadrilaterals instead of triangles.
`a(n)` is the number of simple quadrilaterals on `3\times n` grid. `b(n)` is the number of convex quadrilaterals on `3\times n` grid.
Clearly, `a(1)=b(1)=0` and `a(2)=b(2)=9`.
Find `a(n)` and `b(n)`.