[ (1 / 36) - (1 / x²) ] / [ (1 / 6) + (1 / x) ]
[ (x² - 36) / 36x² ] / [ (x + 6) / 6x ]
remember that:
x² - 36 = (x + 6)(x - 6)
so
[ (x+6)(x-6) / 36x² ] / [ (x + 6) / 6x ]
[ (x+6)(x-6) / 36x² ] * [ 6x / (x + 6) ]
6x / 36x² = 1 / 6x
[ (x+6)(x-6) / 6x ] * [ 1 / (x+6) ] -------------------- > (x - 6) / 6x
The answer is (x - 6) / 6x