Reference these words to words that are in your question;
Density or Specific Gravity can be used to answer the question.
[liter] is not a unit of weight. [liter] is a unit of volume. A unit of weight is pound [lbs]. Density has units of pounds per volume [lbs/volume]. Your answer is simply volume [liters] times density which results weight [lbs]. Which means density must be in units of [lb/liter].
PS:
Convert your density units into [lbs/liter]. In all courses be sure to watch the units. Units do what the numbers do and vice-versa.
As a side note this word problem wants you to use algebra with units, both of which are wonderful in understanding our complicated world.
Curt