If a certain mass has its velocity changed from 6.00 m/s to 7.50 m/s when a 3.00 n force acts for 4.00 seconds, find the mass of the moving object.

By definition we have that the force for time is equal to the product of the mass for the change in speed.
 We have then that
 F * (delta t) = m * (delta v)
 Clearing the mass
 m = (F * (delta t)) / (delta v)
 Substituting the values
 m = ((3.00) * (4.00)) / (7.50-6.00) = 8
 answer
 The mass of the moving object is 8Kg

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