Isnt This Easier!??

sure it will work , if your motor and wheel sprockets line up .
to me the biggest reason for a jackshaft is to get from point A ( your motor sprocket ) to point B ( your wheel sprocket ) which may not be in line with each other , or even on the same side of the bike .

llll said:

for my gear ratio which is 1:12 cant i just use 6:72 sprockets or will i have to take off the pedal assembly and make a jackshaft 4:1 3:1

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What are you going to use the 6T sprocket on? If you are going to mount it on a shaft, 9T might be the practical minimum. I would also be suspect of chain and sprocket life when using such a small counter sprocket on a motor vehicle. 1:6 or 1:7 is good design for single chain reductions. Even up to 1:10 might be possible but a double reduction is preferred at these ratios. For a 12:1 ratio, I would use a jackshaft or gear reduction primary.

I couldn't agree more Scotchmo, Sprockets that small put violent stresses on the chain at high speeds and small sprockets wear quickly. That's the method they use to get one stage reduction on pocketbikes. They went to a smaller pitch chain so the smallest sprocket has a couple more teeth at least, and is still able to get a good amount of reduction.

You don't have to lose your pedals if you place the jackshaft behind the seattube above the bottom bracket (crank bearing housing). Give us pictures or more details about your frame.