Right now you have a/3 = 7. You cannot know what a is yet because it is being divided by 3.
You just need to figure out a way of getting rid of that 3 so you just have a by itself. The way you can get rid of the 3 is by multiplying a/3 by 3, as that will just leave you with a (multiplying is the opposite of dividing so it cancels out).
However, as you’re multiplying a/3 by 3, you also have to multiply 7 by 3 (which is 21) as you have to do the multiplication on both sides of the equation.
Therefore, you’re left with a = 21