9-a^2\3a-9 при a=2\7

9-a^2\3a-9 = (3 - a)(3 + a) / 3(a - 3) = - (a - 3)(a +3)/3(a - 3) = -(a+3) / 3=
= -( 2/7 + 3) / 3 = - 29/7  : 3 = - 29/7 * 1/3 = - 29/21