need to re-glue formica edge of old bathroom vanity. The strip is
about 6' long, and about 2' of one end has come loose. how do I
repair? should i pull the whole strip off and re-glue or just glue
the portion that has come loose? how do i remove old contact cement?

You'll almost certainly never get the piece off w/o breaking it unless
the rest of the adhesive is also letting loose.

You might try very carefully using a putty knife or the ilk and see if
can actually separate the piece by pushing it under and slicing the rest
of the sheet loose but I'll wager odds are not good.

I would simply recoat (both surfaces) after removing an sizable pieces
of dirt grit and reglue w/ contact cement. You'll want to keep the two
surfaces from touching until it dries w/ some thin stickers, of course,
and then press back down from the inside edge(s) outward to avoid any
buckling.

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