Question 871498
Substitute {{{u=d^2}}}
{{{9u^2-490u+490=0}}}
{{{9(u^2-(490/9)u+(245/9)^2)+490=9(245/9)^2}}}
{{{9(u-(245/9))^2=9(245/9)^2-490}}}
{{{(u-245/9)^2=(245/9)^2-490/9}}}
{{{(u-245/9)^2=60025/81-4410-81}}}
{{{(u-245/9)^2=55615/81}}}
{{{u-245/9=0 +- sqrt(55615)/9}}}
{{{u=245/9 +- (7/9)sqrt(1135)}}}
{{{d^2=245/9 +- (7/9)sqrt(1135)}}}
Both values are positive.
{{{d=sqrt(245/9 +- (7/9)sqrt(1135)))}}}