By conservation of linear amount of movement we have:
m1v1i + m2v2i = m1v1f + m2v2f
Where,
v1i = v2i = 0 (assuming initial rest)
m1v1f + m2v2f = 0
Substituting the values:
m1 (-3) + (1) (150) = 0
Clearing m1:
m1 = (- (1) * (150)) / (- 3)
m1 = 50 Kg
answer:
his mass is 50 Kg