Can you help me solve this?(Not homework)

This is not homework,is just a problem which is the subject of an argument I have with my professor.Can you please help?
A cube is thrown under the water with the speed of 1m/s.After 0.4 s the cube has traveled 1m.Knowing the density of water find the density of the cube.
I say that this is solved using notions of acceleration which are over what is learned in my grade and my professor say it’s a very easy problem solved using only mechanics of fluids and energy variation laws but she didn’t tell me how to actually solved it.
Can you help me please?