Hi Autumn! So to find unit rate you are looking for price per one unit of whatever object is mentioned in the problem (in this case, that would be gallons of milk). I think the easiest way to solve these types of problems is to set up a proportion.
$9.75 = x
3 gallons 1 gallon
For this problem the unit rate (also called unit price when you are dealing with money) would be $3.25/gallon.