Question 68800
LCD is the common denominator between each fraction that makes it possible to add and subtract. For instance to add 1/2 and 1/4, the LCD is 4 since 2/4 and 1/4 can be added. For this problem, you need to factor the left fraction first
{{{(7)/(a^2 + 2a - 63)}}}
{{{(7)/((a+9)(a-7)))}}} If you FOIL (a+9)(a-7) you get a^2 + 2a - 63
You will then see that (a+9) is a common factor, so to complete the process, you need to multiply both sides by (a-7)/(a-7). Now you should see the LCD is (a+9)(a-7).