I'm like a snail I've got
To formulate a plot,
Snails: beleaguered schemers of the mollusk world
Snails scientifically formulate (in this case meaning create) their shell. Therefore, like snails make their shells, Em needs to make a plan to escape from 8 mile. It’s also a reference to the former line, where he says he feels as if he’s stuck in one spot; snails take a long time to move.
A plot also refers to a plot of land or home. Therefore Em could be saying he has to form his home in rap and make sure he sticks to it. Just like a snail makes their own home on their backs.
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