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:** $$\boxed{-160}$$