3 1/4 pounds is the same as 13/4 pounds as a mixed number (you multiply 4 times 3 and add the 1 in the numerator and place it all over the denominator).
13/4 pounds X (16 ounces / 1 pound) = 52 ounces
Notice how the pound units cancel out, so I know I have the conversion correct in case you get confused if pounds goes on top or on the bottom.