The method shown below is not synthetic division. I will correct this article, eventually. We don't know their values, because we don't know the value of x. But we can do arithmetic with them anyway. I suppose you see how that is done.
Subtraction is about the same.
Be careful with the signs. It is a lot like normal multiplication. If x were 10, then this would be normal decimal multiplication. Anyway, you don't have to write it in columns, in order to multiply. But it may be easier for you.