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