First you must know that what’s outside of the square root is called the coefficient, and it’s the little number outside of the box area.
Next, the number inside the square root or box is called the radicand.
To add radicals you must add coefficients to coefficients, and radicands to radicands.
In your problem the three, which is the radicand, is added to the other 3, which is also the other radicand.
The two outside, which is the coefficient is left outside. The reason the answer has a three outside is that any radical without a number outside is assumed to have the coefficient of one automatically.
One important rule to always remember when adding radicals is that the radicand must be the same number to the other radicand to be added.
I know it can be a bit confusing, but I hope this helped a little.
