OK, Omar, I promise this is easier than you think. A variable is in place of a number. At first, you were given _ + 5 = 10 and asked to fill in the blank. You would use your fact families and know that 10 - 5 = 5 so the blank is 5.
Instead, you are given a + 5 = 10 and asked to solve for a. Again, you know 10 - 5 = a, so a = 5.
Or, you might be asked to solve a + 5 if a = 6. You replace a with 6. 6 + 5 = 11, so the answer is 11.
If not given a variable to work with, you can pretty much use any letter you want. Trying to find the pounds of apples and peaches sold? Use a and p.
Seven times something. Well, that's simple, but times what? The result of subtraction is the difference, so we're subtracting something. In this case, y and 1, so y - 1. The difference of 1 and y would be 1 - y.
Now, here it can be confusing. We are multiplying 7 times the difference. You might say that's 7y - 1, but that's only multiplying 7 times y and not the whole difference.
7(y - 1)
It doesn't ask you to go any further, but we would then multiply this out.
7(y) + 7(-1) = 7y - 7
Let's use an actual number instead of y. How about 6?
7 times the difference of 6 and 1. If we wrote that out as 7(6) - 1, that equals 41. If we do 7(6 - 1), we are taught that's 7(5) = 35, but we can also do 7(6) + 7(-1) = 42 - 7 = 35 and get the same result.
Hope this helps!