(x+3)/(x-3)-x/(x+3)=72/(x^2-9)

(x+3)/(x-3)-x/(x+3)=72/(x^2-9)

(x+3)/(x-3)-x/(x+3)-72/((x+3)(x-3))=0

((x+3)(x+3)-x(x-3)-72)/((x+3)(x-3)) = 0

(х²+6х+9-х²+3х-72)/(х²-9) = 0

(9х-63)/(х²-9) = 0

9х-63=0

9х=63

х = 7

Ответ. х =7.