Answer:

 

Nine Hamburgers

 

Explanation:

 

If two boys eat three hamburgers in two minutes, each boy is eating 1.5 hamburgers in two minutes (3 hamburgers / 2 boys = 1.5 hamburgers).
So then in two minutes, six boys would eat 1.5 hamburgers each and would eat nine hamburgers total (6 boys x 1.5 hamburgers = 9 hamburgers).

3 hamburgers / 2 boys = 1.5 hamburgers
6 boys x 1.5 hamburgers = 9 hamburgers