Question 31400
First thing to do is factor the bottom for both
1/(x^2-3x-28)==>1/(x+7)(x+4)
x/(x^2+6x+8)==>x/(x+4)(x+2)

To find the common denominator find the things that are similar and disimilar. The things that are disimilar need to be added to the bottom.

(x+4) is in common
(x+7), (x+2) aren't so add them to the denominator

(x+4)(x+7)(x+2) is the lcd.