To find the number that added to -70 has a sum of zero, you can set up the equation:
\[ x + (-70) = 0 \]
This simplifies to:
\[ x - 70 = 0 \]
To solve for \(x\), add 70 to both sides:
\[ x = 70 \]
So, the number that needs to be added to -70 to achieve a sum of zero is 70.