Answer: 3
Step-by-step explanation: To find the sum of these two numbers, remember that adding a negative is exactly the same thing as subtracting a positive. In other words, when we see 18 + (-15), we can change the problem so it's easier to understand and we will change in to 18 - 15.
Now, this is just basic subtraction.
18 - 15 = 3
Therefore, 18 + (-15) = 3