Hello! 
When an equation adds a negative number (and the + and - signs are directly next to each other), the + is always turned to a minus. 
In this case, the 6+(-9) would simply turn into 6-9. 
Because 6 is subtracting a larger number than itself, you know that the final answer will be a negative number. 
6-9= -3
The correct answer to this question is -3.
I hope this helps you!