Hi there! 
Here is what the rules says 
+ * + = + (Plus times plus = plus)
+*- = - (Plus times minus = minus)
- * - = + (Minus times minus = Plus
- * + = - (minus times plus = minus)
You need to know the rules by heart because you will use it a lot. 
So let see how it works with the problem 
6 -(-7)
What does the rules say? It says 
- * - = plus 
So, we gonna use PLUS for the sign 
6-(-7) = 6 + 7 = 13 
13 is the correct answer. 
I hope I helped. Please let me know if you have more questions.