Let's solve these expressions step by step:

a) $-3 + (-8)$: $-3 + (-8) = -3 - 8 = -11$

b) $-9 + 12$: $-9 + 12 = 12 - 9 = 3$

c) $-17 + (-40) + (-18)$: $-17 + (-40) = -17 - 40 = -57$ $-57 + (-18) = -57 - 18 = -75$

d) $20 + 30 + (-10)$: $20 + 30 = 50$ $50 + (-10) = 50 - 10 = 40$

Final Answers:

a) $-11$
b) $3$
c) $-75$
d) $40$

Would you like more details on any part of the solution?

Here are some related questions to explore further:

1. What is the result of $-15 + (-5)$?
2. How does the addition of negative numbers affect the final sum?
3. What is the sum of $-20 + 30 + (-15)$?
4. How would you solve $-25 + 25 + (-50)$?
5. What is the significance of zero in addition when combining positive and negative numbers?

Tip: When adding a negative number, it's the same as subtracting the corresponding positive value.