Answer
2500 m/s.
Work Step by Step
The momentum in the horizontal direction will be conserved. Let the positive direction be the direction in which the nucleus was originally moving.
$$ (222 u)(320 m/s) = (218 u)(280 m/s) + (4.0 u)v$$
Solve for velocity v = 2500 m/s, rounded to 2 significant figures.
