The problem is asking as to perform an addition of signed numbers.
The firs one to add is 3 and the other one is -7.
We can understand the meaning of this type of addition by using the number line forst, and then have a very simple "short cut" every time we fce problems like this.
The number line approach:
locate yourself at the mark "3" on the number line, and then add the number "-7" whichmeans go to the left (as the negative indicates) 7 units. You will see that you move through zero, and then land on the number "-4".