Variable-variable=junk

im trying to subtart one quantity from another then stick the answer in to an update statment, but all i get is an error in syntax message

variable math bit=
qty=request.form("oqty")-Request.form("qty")

sql =
sSQL="Update spares SET spares.location='"&request("location")&_
"',spares.machine='"&request("machine")&_
"',spares.part_desc='"&request("part_desc")&_
"',spares.part_number='"&request("part_number")&_
"',spares.qty='(" & qty & ") "&_
"',spares.min_qty='"&request("min_qty")&_
"',spares.updated_by='"&session("name")&"' WHERE(spares.id)=" & form_id