First, because "F= ma" you can ignore the "speed v= 7 m/s". Force depends on acceleration, not speed. You are told that the acceleration is 3 m/s^2, negative because it is downward. If the elevator were standing still the "force" on each brick would be its weight, mg, where g= 9.81 m/s^2 the "acceleration due to gravity". While accelerating downward at 3 m/s^2, F= ma= m(9.81 3)= m(6.81).
The mass, m, is given in kg while the acceleration is given in m/s^2 so the force, ma, is given in "kg m/s^2" or "Newtons".
