Question 925613
Roots, {{{x=(12+- sqrt(144+4*P))/2}}}, using formula for general solution of quadratic equations;


{{{x=(12+- 2sqrt(72+P))/2}}}


{{{x=6+- sqrt(72+P)}}}


The condition about the roots means you could try  {{{highlight_green(6+sqrt(P+72)=(6-sqrt(P+72))^2)}}}.  Work from that, simplifying and then solve for P.