One rule in algebra is what you do to one side, you do it to other side. So if you multiply a number in one side, multiply the same number in other side. Here in this question, you are trying to find the value of the variable u. Variable is called so because the value of it varies depending on different question. Here u is going to be a constant number which when multipled by 3 and then subtracted by 3 equals 15.
So first step is we try to get constants on one side. So we add 3 on both sides to get rid of 3 on left.
3u - 3 + 3= 15+3
3u= 18
Now we divide by 3 on both sides to get u by itself.
