Question 709647
I think you need to know that the diameter of earth is about 8000 mi. (radius about 4000 mi.) 
The volume of a sphere is
(1) V = 4/3*pi*(r^3)
The ratio of Ve/Vm is
(2) (re)^3/(rm)^3 = 6.7
Take the cube root of (2) and get
(4) re/rm = 1.885 or 
(5) rm = 0.530*re or
(6) dm = .0530*8000 or
(7) dm = 4244 mi.
Answer: Mars' diameter is approximately 4144 miles.