We know the relation between force and mass and acceleration follows this relation: F = m * a
And we can calculate the acceleration as:
Vf = Vi + a * t
where Vf is final speed, Vi is initial speed, a is acceleration and t is time
7.5 = 6 + a * 4
7.5 - 6 = a * 4
1.5 / 4 = a
a= 0.375 m2/s
The the mass is F = m * a 3 = m * 0.375 m = 8
Then the mass is 8 kg