Diamond patterns III

This task is about spatial patterns and rules.
Miriam began making the diamond pattern below with matches.

She noticed a relationship between the number of diamonds and the total number of matches used, which she recorded on a chart.

Number of diamonds

1

2

3

4

5

6

7

8

Number of matches

4

7

           
 
a)
 
Complete the table above to show the number of matches used, for each diamond pattern.
  
b)
How many matches would be needed for 20 diamonds? __________ matches.
You may show your working in the space below.
 
    

    

    

    

    

    

 
c)
 
What rule did you use to find the number of matches needed for 20 diamonds?