The perimeter of a triangle is 42 yards. the first side is 5 yards less than the second side, and the third side is 2 yards less than the first side. what is the length of each side?

The 1st side is 13 yards, the 2nd is 18 yards, and the 3rd is 11 yards

So, let's start off with what you know. 
You know that the perimiter of the triangle is 42 yards.
You know that the first side is 5 yards less than the second.
You know that the third side is 2 yards less than the first.
So, we can make the second side x.
We come up with something like this : 

Second Side : x
First Side : x-5
Third Side : x-7

Now we make an equation.

x+x+x-5-7 = 42
3x-12 = 42
    +12    +12
3x      =  54
     3x = 54
       x = 18
So the second side is 18 yards.
Then we can find the First side.
First side is 13 yards.
Then the third side.
The Third side is 11 yards.
So the lengths are : 

First Side : 13 Yards
Second Side : 18 Yards
Third Side : 11 Yards.

I hope I was able to help!

What mathematical statement is missing ? ? the answer is b