3/4. Because you first change 3 1/4 in a improper fraction which would be 13/4 then your gonna make the denominators the same so 15/6 times 4 = 60/24. And. 13/4 times 6 = 78/24. Then your going to subtract 78 and 60 which is 18 so you have 18/24 and you can simplify that into 3/4 (does that make any sense)