In many instances while tutoring Algebra students I have seen that students do not distribute the negative as they are supposed to. This is because they do not understand what the distributive property is. For example, many students will to the following:
3x - (x +2)
3x - x +2
This is incorrect. The student did not distribute the negative. The distributive property tells us that we must multiply both the x and the 2 by -1, because that is in fact what that negative means. Thus,
3x - (x+2)
3x - x - 2
2x - 2
This is the correct way to solve it. Remembering that the negative must be distributed to everything in the parenthesis is critical.