Line y = 2x – b cuts the parabola $\mathrm{y}={\mathrm{x}}^2-4\mathrm{x}$ at points A and B. Then the value of b for which $\angle$AOB is a right angle is

Line $\frac{2\mathrm{x}-\mathrm{y}}{\mathrm{b}}=1$, Homogenising parabola with line ${\mathrm{x}}^2-4\mathrm{x}\left(\frac{2\mathrm{x}-\mathrm{y}}{\mathrm{b}}\right)-\mathrm{y}\left(\frac{2\mathrm{x}-\mathrm{y}}{\mathrm{b}}\right)=0$ 
Given $\angle$AOB = 90⁰Coefficient of x² + Coefficient of y²= 0
$1-\frac{8}{b}+\frac{1}{b}=0\Rightarrow b=7$