Answer:
The answer is 4 \sqrt{7}.
Explanation:
The formula is a\sqrt{x} + b\sqrt{x} = (a+b)\sqrt{x}, but if you don't understand it, here's an easier way to think about it.
You can think of \sqrt{7} as x (or any other variable). Now, the equation is 3x + x. If you've ever done algebra, (you probably have if square roots popped up on your homework) you know that this equals to 4x. Now, replace the x with \sqrt{7}, because we already decided that those two were equal. Therefore, the answer is
4 \sqrt{7}.