Here's the expression based on your question:
2√7x + √7x - 3√7x - 7 =
3√7x - 3√7x - 7 =
0-7 =
-7
The correct answer is -7. You can easily subtract 2√7x, √7x, and 3√7x because they all have √7x in common, so it's basically the same as subtracting simple numbers 2, 1, and 3.
