Answer
0.70 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 tackler was originally moving.
$$(110 kg)(2.5 m/s) + (82 kg)(-5.0 m/s) = (110 kg + 82 kg)v$$
Solve for v = -0.70 m/s.
The negative sign means they are moving in the direction that the halfback was originally running.
