It's just the same as if you have a minus. In the video, he had 3 square root 3 plus 2 square root 3 to get 5 square root 3. If you have something like -4 square root 75 plus 10 square root 75, you could combine those to 6 square root 75. In your case, you should also think about what's inside the square root. Because 75 is 3 *25, and you can take the square root of 25 to get 5, and simplify further that way. For instance, your -4 square root 75 is the same as -4 times 5 times square root three, or -20 times square root 3.
