Total height of lumber, H = 10 1/2 feet = 21/2 feet .
Height of side panel, h = 5 2/3 feet = 17/3 feet .
Now,
Extra lumber required, L = 2 × Height of side panel - Total height of lumber
![L=[2* ((17)/(3))]-(21)/(2)\\\\L = (5)/(6)\ feet](https://img.qammunity.org/2021/formulas/mathematics/high-school/6c62kzsv0clowcfdwr896z8qnjf9zzq2cx.png)
Therefore, extra lumber required is
feet.
Hence, this is the required solution.