You want to find a common denominator and the easiest way to do this is to find the least common denominator which would be 36 in this case. To get the denominators to 36 you can multiply until you get a shared denominator. 12*3=36 and 9*4=36 so now you want your numerators to match so multiply by each corresponding number so 11*3=33 and 7*4=28 so you’d get something like 33/36-28/36=5/36 which would be your answer. Sorry if this is wrong, it’s been a while since I’ve done this.