Slope of $OP$ $=\frac{k}{h}\Rightarrow$ slope of the tangent is $\frac{-h}{k}$

Hence, the equation of tangent $PQ$ is $\frac{(y-k)}{x-h}=\frac{-h}{k}$
$\Rightarrow$ $y=-\frac{h}{k}x+\frac{h^2+k^2}{k}$
$\Rightarrow {\left(\frac{h^2+k^2}{k}\right)}^2=40{\left(\frac{-h}{k}\right)}^2+10 \left\{\begin{array}{l}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{o}\mathrm{f} \mathrm{ta}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y} \mathrm{is}\\ c^2=a^2{m}^2+b^2\end{array}\right\}$
$\Rightarrow {\left(x^2+y^2\right)}^2=40x^2+10y^2$
$\Rightarrow a–b=30$