\textup{A 2 x 7 rectangle has tiling with 1 x 1 and 1 x 2 tiles (singletons and doubletons).}
\textup{How many such tilings of a 2 x 7 grid are there?}

\textup{Let }a_{n}\textup{ be the number of tilings of a 2 x n grid using 1 x 1 and 1 x 2 tiles so that the}
\textup{two rightmost squares are occupied by singletons, let }b_{n}\textup{ be the number of tilings}
\textup{ with one singleton and one doubleton in the rightmost squares, and let }c_{n}\textup{ be the}
\textup{ number of other tilings (two horizontal doubletons or one vertical doubleton). The}
\textup{problem asks for }a_{7}+b_{7}+c_{7}. \textup{ When one appends another column to the right side,}
\textup{forming a 2 x (n + 1) grid, either one can add 2 singletons or a vertical doubleton, or}
\textup{one can change any singletons in the nth column to doubletons. This yields:}


***doubletons may be placed vertically or horizontally.

Use the recurrence relations to obtain a numerical answer to the problem.

Not sure how do start this. Any help?