A dock worker applies a constant horizontal force of 77.0 to a block of ice on a smooth horizontal floor. The frictional force is negligible. The block starts from rest and moves 9.00 in the first 7.00 .

What is the mass of the block of ice?

Do it in steps.

First compute the acceleration rate "a" from:
Average velocity = 9 m/7 s = 1.286 m/s
Final velocity = 2 x (average velocity)
Acceleration = (Final velocity)/7 s

Mass = Force/Acceleration