Do you have the correct answer? My answer seems too low..unless this particular game of football is between large men and small children... I got 36.6kg

Momentum = Mass * Velocity

M1 = 110 * 4 kg/ms

M1 = 440

Law of conservation of energy in a closed system means momentum must remain constant.

M2 = M1

440 = M2 * V2

440 = 3(x+110) where x is the unknown players mass)

x = 36.6kg