$10.$ In the figure below, the two rectangles $($and their parts$)$ are parallel and 'overlapping'. Determine the shape factor $\backslash ($ F$\_\{1$ a$-2\}\backslash ),$ where surface $2$ is the union of $\backslash (2$ a $\backslash )$ and $\backslash (2$ b $\backslash ).$ The result should use shape factors only for parallel overlapping rectangles.