you can use the formula:
(m1 + m2)vf = m1vi
m1 = mass of first rugby player (75 kg)
m2 = mass of second rugby player (90)
vf = their final velocity
vi = the initial velocity
(m1 + m2)vf = m1vi
(75 + 90)vf = (75)(8.0)
165(vf) = 600
vf = 3.64 m/s
hope this helps :)