Suppose you have a large number like 684, and you need to divide or factor it. You can look at the number and get a few clues that will help you. First, recognizing that the number is even, you know it's divisible by 2. But guess what? If you add the sum of numbers together, and they equal a multiple of 3, then the number is divisible by 3. In this case, 684 (6+8+4) equals 18, which is divisible by 3, so 684 can be divided equally by 3. So, we know that it is divisible by 2 and 3, and if you multiple 2 times 3, you get 6. 684 can also be equally divided by 6. Try this with other numbers and see what you get. It certainly helped me!
It's a good idea to be neat when doing algebra. If you are having difficulty adding or subtracting horizontally, try doing it vertically, lining up the like terms. A lot of math teachers will give at least partial credit if part of the equation is correct. Also, when your work is neat, you will be more apt to locate your errors prior to submitting your work. I demonstrate this neatness in the videos that I have posted on my page. Take a look at them.