-15 + (-3)

-15 + (-3) = -18

To calculate the sum of two negative numbers, you can follow these steps:

1. Start by writing down the two numbers:

-15
-3

2. Add the two numbers together:

-15 + (-3) = -18

So, the sum of -15 and -3 is -18.

To calculate -15 + (-3), you simply add the two numbers together.

-15 + (-3) = -18

The addition of negative numbers works by using the concept of subtraction. In this case, adding -15 and -3 is equivalent to subtracting 3 from -15.

So, -15 + (-3) is the same as -15 - 3, which equals -18.