Working out a formula from a set of data

Hi all.

I would like to know what sort of formula my competitiors (if any) are using to price up their products. Some data I have gathered is below.

my cost ex VAT Competitors Retail price

2.56 13.5

7.88 21.5

14.48 29.98

15.93 34.17

18.33 39.16

20.81 45.95

25.25 52.17

What I would like to know is how do I workout the formula that best suits the relationship between cost of product and final retail price.

Or could someone tell me what maths subject this comes under so I can do some reading up and learn myself.

Any help would be greatyly appreciated and a big help.

Thanks!

Dave