1  3  5  7
3  7 15 27
5 15 37 79 
7 27 79  x

Let's see how that was built:

Start with the first four odd numbers 1,3,5,7 going
across and down, like this:

1  3  5  7
3  
5  
7 

Now we complete a 2x2 square by adding 3 corners of
the square and putting the sum in the lower right corner
of the square, like this: 1+3+3=7

1  3  5  7
3  7 
5  
7 

Now 3+5+7=15

1  3  5  7
3  7 15 
5  
7

Now 3+7+5=15

1  3  5  7
3  7 15 
5 15  
7

Now 5+7+15=27

1  3  5  7
3  7 15 27 
5 15  
7

Now 5+15+7=27

1  3  5  7
3  7 15 27 
5 15  
7 27

Now 7+15+15=37

1  3  5  7
3  7 15 27 
5 15 37  
7 27

Now 15+27+37=79

1  3  5  7
3  7 15 27 
5 15 37 79  
7 27

Now 15+37+27=79

1  3  5  7
3  7 15 27 
5 15 37 79  
7 27 79

Now what?  You finish :)

Edwin