Преобразуем выражение в скобках:
(6a+c)/(a²-6ac)+(6a-c)/(a²+6ac)=(6a+c)/(a(a-6c))+(6a-c)/(a(a+6c))=
=((6a+c)(a+6c)+(a-6c)(6a-c))/(a(a-6c)(a+6c))=
=(6a²+37ac+6c²+6a²-37ac+6c²)/(a(a²-36c²))=12(a²+c²)/(a(a²-36c²).
12(a²+c²)(a²-36c²)/(a(a²-36c²)((a²+c²))=12/a.