As a general rule, 1 lb of LME in a gallon of water yields a wort of 1.036.

As a general rule, 1 lb of DME in a gallon of water yields a wort of 1.044.

So, to convert back and forth, you multiply by 36/44 to go from LME to DME and multiply by 44/36 to go from DME to LME.

So, 6lbs LME is roughly equivalent to 6*36/44 = 6*0.818 = 4.90 lbs of DME.

likewise, 6lbs DME is roughly equivalent to 6*44/36 = 6*1.22 = 7.33 lbs of LME.

You can substitute the ACTUAL figures based on the manufacturer of the extract that you are converting, though these numbers are close enough and you'll end up rounding off anyways.

Hope this helps