To find the number that, when added to \(-70\), results in a sum of zero, you can set up the equation:
\[ x + (-70) = 0 \]
This can be rewritten as:
\[ x - 70 = 0 \]
To isolate \(x\), add \(70\) to both sides:
\[ x = 70 \]
So, the number is 70.