First you have to find common denominators to subtract them. To do that, just find a number that both 4 and 10 can go into. The smallest number is twenty.
So to convert 3/4 to a fraction with a denominator of twenty, you have to find what you multiply 4 by to get 20, which is 5. So you then multiply 3 by 5 and and 15/20.
3/4=15/20
You do the same thing with 3/10. Find what you multiply 10 by to get 20, which is 2. So you multiply 3 by two.
3/10=6/20
You can now subtract the 6 from 15 and get 9, so the difference is 9/20.
