The goal is to get y by itself. Don’t be fooled by the fractions. This problem is very similar to x+4=10.
We want to get rid of the 4/5. Well, not actually get rid of it, but move it to the other side of the equal sign. The way we do that is to subtract it from both sides.
On the left side, 4/5-4/5 cancel out.
y = 3/4 -4/5
Now, to subtract fractions, you need a common denominator. One common denominator is 4*5=20. Others are possible. If you use the lowest common denominator, there will be fewer steps later on.
Remember that 5/5 and 4/4 are both equal to 1. You can multiply anything by 1 and not change it. 1 is special that way.
y = (3/4) –(4/5) (Just rewritten for clarity)
y = (5/5)*(3/4) –(4/5)
That 5/5 doesn’t really change anything, but lets us change the denominator.
y = (15/20) –(4/5)
y = (15/20) –(4/4)*(4/5)
y = (15/20) –(16/20)
Now we have a common denominator and can subtract.
y = -1/20
This kind of problem can be confusing. There are 2 principles at work. First, you can multiply by 1 and not change anything. Second, you need a common denominator to add or subtract fractions. When we changed the denominators, like when we made 3/4 into 15/20, we didn’t change its value. Both are the same. The trick for getting that common denominator is to multiply one term by a special form of 1, like 4/4 or 5/5. The choice here is to use the denominator of the other term. For this problem, we converted 3/4 into 20ths using the 5/5 because 5 was in the denominator of the 4/5 term. And we used 4/4 to convert 4/5 into 20ths because 4 was in the denominator of the 3/4 term.
I hope this helps. You can use the comment feature to ask more questions if this is confusing.