What is the density of lithium metal (in g/cm3) if a cube measuring 0.82 cm × 1.45 cm × 1.25 cm has a mass of 0.794 g?

Density
= Mass / Volume
= (0.794 g) / (0.82 × 1.45 × 1.25 cm³)
= 0.534 g/cm³

let's see... 
.. (1) you just joined YA
.. (2) you posted 1 question.. a trivial density question
.. (3) the Troll who's been menacing the YA chemistry 
.. .. . section has posted nearly identical questions
.. . . .over the past couple of months over and over again. 
.. . . .Each time via a newly created account with a female
.. . ...screen name of 5 or 6 letters, 1 word, all lowercase
.. . .. JUST LIKE YOURS. 
<sigh>

You sure sound an awful lot like a common troll.  Especially since you had a choice.. (1) EASILY calculate density from the data, or (2) turn on your PC, create a brand new YA account, post this 1 question, wait for someone else to solve it.

I'm going with TROLL. 

cubes have equal sides, I'll assume you mean rectangular solid.

volume = 0.82x1.45x1.35 = ____ cm³
density is mass / volume = 0.7940.82x1.45x1.35 = ____ g/cm³