Question 700457
{{{((x-5))/((x^2-36))}}} - {{{((x-7))/((x^2-x-42))}}} 
Factor both denominators
{{{((x-5))/((x+6)(x-6))}}} - {{{((x-7))/((x-7)(x+6))}}}
Cancel out (x-7)
{{{((x-5))/((x+6)(x-6))}}} - {{{1/((x+6))}}} 
The common denominator; (x+6)(x-6)
{{{((x-5)-(x-6))/((x+6)(x-6))}}} = {{{((x-5-x+6))/((x+6)(x-6))}}} = {{{1/((x+6)(x-6))}}}