Assuming you meant to write [tex] \dfrac{7x+42}{x^2+13x+42} [/tex], then you can simplify this by first factoring the numerator and denominator.
[tex]\dfrac{7(x+6)}{(x+6)(x+7)} [/tex]
Notice that (x+6) is common to the numerator and denominator. You can cancel this factor so long as you remember to exclude 6 from the domain.
Simplifying further, you have [tex]\dfrac{7}{x+7} [/tex], where x ≠ 6, x ≠ 7.
