X^2=t t^2-(9a^2+4)^2+36a^2=0 t=(9a^2+4+-|9a^2-4|)/2 9a^2<4 (-2/3;2/3)<br>t=(9a^2+4+-(4-9a^2))/2 t1=4 x1=2 x2=-2 t2=9a^2 x3=3a x4=-3a