1. **State the problem:** Calculate the sum of $-70$ and $-90$.
2. **Formula and rules:** When adding two negative numbers, add their absolute values and keep the negative sign.
3. **Intermediate work:**
$$|-70| = 70, \quad |-90| = 90$$
$$70 + 90 = 160$$
4. **Apply the sign:** Since both numbers are negative, the result is negative:
$$-70 + (-90) = -160$$
5. **Final answer:**
$$\boxed{-160}$$
Add Negative Numbers
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