This is an example of an inelastic collision (since the players remain joined after the collision), in which kinetic energy is lost. However, linear momentum is conserved since the net external forces equals zero.
momentum before collision = momentum after velocity
to be more specific...
momentumfull + momentumline = momentumboth
Since momentum (p) = mass (m) times velocity (v), you can say that:
mfullvfull + mlinevline = mbothvboth
Solve for vboth:
v
both = (m
fullv
full + m
linev
line)/m
both
vboth = (90 kg x -6 m/s + 110 kg x 4 m/s) / (90 kg +110 kg) = -0.5 m/s
Keep in mind that mboth equals the sum of the players since they become one object after the collision. Also, be sure to set one of the players' velocity as negative and one positive since they start off running in opposite directions. The sign of the answer (negative) implies that they are going in the direction of the fullback (negative). If you set the linebacker as negative, then your answer will come out positive.
To sum it up: The two players, joined together, travel at a velocty of 0.5 m/s in the direction the fullback was initially travelling.
Hope this helps!