Hey there!
First, we need to get a common denominator. In order to do this, we need to use the least common multiple. The least common multiple of 4 and 6 is 12. That means that we have to multiply 5/6 by 2/2 and 3/4 by 3/3. This is how we change the fraction into one that is equivalent but with a different denominator because essentially we are multiplying it by one because 2/2 and 3/3 are equal to one. We have:
5/6*2/2 =10/12
3/4*3/3 = 9/12
10/12 - 9/12 = 1/12
Your answer is 1/12.
Hope this helps!