Answer:
Step-by-step explanation:
Two numbers are given.  -7 and 3.  What are happening with the numbers? well, the question STARTS with -7 points then  he gains 3.  Well what happens if you had 0 points and gained 3?  you'd add them.  So the same thing happens here.  so the expression is -7+3 = -4
In a proper sentence the answer woud be something like "The final score is -4 since he gained three after having -7."
Let me know if there was something you didn't understand.