The width should be 100.8 inches since you multiply 42 × 2.4. Hope it helps.
Easy peasy, this is a representation of differnce of 2 perfect squares
a^2-b^2=(a-b)(a+b)
Since all the sides add up to 10x+2, that means that 10x-2-<side1>-<side2>=<side3>. Plugging it in, we have 10x-2-5x-(2x+9)=side3, and 5x-2-(2x+9)=side3, then expanding it to 5x-2-2x-9=3x-11=side3
