Well you can try to think of the question like this:
? -(-7) = 5
If they are 2 negative signs beside each other like in this question, it becomes positive.
? - (-7) = 5
? --7 = 5
? + 7 = 5
Then solve for the ?. By rearranging the equation to get ? On one side of the equation.
? + 7 = 5
? + 7 -7 = 5-7
? = 5-7
? = -2.