6m x 2m = 12m
4m x 2m = 8m

Explanation; we used ' 2 ' because he wanted to double the are of his garden...
                 ; double means 2
                 ; so we used 2 to multiply their length and width by the same amount.

I think that is how you solve it... 
It's only my answer... I'm not sure if its correct or not... sorry... i just wanted to help/...