Step-by-step explanation:
Total area of merged figure = area of vertical rectangle + area of horizontal rectangle.
[tex] \therefore \: 3x(2x - 7) + (3x + 4)x = 85 \\ \\ \therefore \: 6 {x}^{2} - 21x+ 3 {x}^{2} + 4x = 85 \\ \\ \therefore \: 6 {x}^{2} + 3 {x}^{2} - 21x + 4x - 85 = 0 \\ \\ \huge \red {\boxed {\therefore \: 9 {x}^{2} - 17x - 85 = 0}} \\ hence \: proved.[/tex]
