We can put the dimensions of Mary's garden into ratio format:
6ft by 8ft = 6ft : 8ft
Now if she increased it by a scale factor of 2=
6ft : 8ft = __ : 16ft
So we can see that 8 was multiplied by 2, so we do the same for 6:
6ft : 8ft = 12ft : 18ft.
So the measurements should be 12ft by 16ft