Question 1071073
{{{(20+sqrt(400-4*2k*21))/(4k)-(20-sqrt(400-4*2k*21))/(4k)=2}}}


{{{(2*sqrt(400-4*2k*21))/(4k)=2}}}


{{{(2*2*sqrt(100-42k))/(4k)=2}}}


{{{sqrt(100-42k)/k=2}}}


square both sides.
{{{(100-42k)/k^2=4}}}


{{{100-42k=4k^2}}}


{{{50-21k=2k^2}}}


{{{highlight_green(2k^2+21k-50=0)}}}


Resorting to general solution of quadratic equation for this ,
{{{k=(-21+- sqrt(21^2+4*2*50))/4}}}


{{{highlight(k=(-21+- 29)/4)}}}-------but I'll leave it to you, to finish this.