Given data
*The mass of Bruce is m_1 = 45 kg
*The initial velocity of the Bruce is u_1 = 2 m/s
*The mass of the biff is m_2 = 90 kg
*The initial velocity of the Biff is u_2 = -7 m/s
*The final velocity of the first glider is v_2 = -1 m/s
According to the law of conservation of linear momentum, the total linear momentum of a system remains constant
Applying the law of conservation of momentum as
Substitute the known values in the above expression as
Hence, the speed of the bruce knock backwards is v_1 = -10 m/s