The formula for the area of a rectangle is [tex] b\cdot h[/tex], so given that [tex]h=5x+3, b=3x-4[/tex], the area of the entire rectangle is [tex](3x-4)(5x+3)=15x^2-20x+9x-12=15x^2-11x-12. [/tex]. We now subtract the white part, with area [tex] 2x*4x=8x^2[/tex], so [tex] 15x^2-11x-12-8x^2=7x^2-11x-12[/tex]
