Question 118640
I think the problem has already been broken down for you.  From what I understand,  you have a triangle with side lengths x, x+1, x+2.  What's important to remember is that those sides do have lengths, we just don't know them.  But someone who does added them up to get a perimeter of 12.  Now we want to see what numbers they added to get that.  We know that x+(x+1)=2x+1, and (2x+1)+(x+2)=3x+3.  So the perimeter of this triangle is 3x+3.  But wait!  We were told the perimeter is 12!  That means 3x+3=12.  Now solve for x: 3x=9, x=3.  So the side lengths are actually x=3, x+1=4, x+2=5.  Notice, 3+4+5=12.
In general, you need to connect all of the information you are given and figure out the missing piece that joins them together.  Here, you had side lengths in terms of x and a number for the perimeter.  You needed to find what x makes that perimeter true.  Good luck!