Simplify the expression -9 + 3.

We have a negative number, $-9$, and a positive number, $+3$. We need to add them.

When adding numbers with different signs, subtract the smaller absolute value from the larger absolute value, and keep the sign of the number with the larger absolute value.
The absolute value of $-9$ is 9.
The absolute value of $+3$ is 3.
Subtract the smaller absolute value from the larger: $9 - 3 = 6$.
Since 9 (from $-9$) has a larger absolute value than 3 (from $+3$), and $-9$ is negative, the result will be negative.

The simplified expression is $-6$.