To get the length in cm, you will start by finding the volume using D=mass/volume. You get 2.70 g/cm3= 7.6 g/vol. that next step is Volume =7.6 /2.70 cm3 which give you a volume of 2.8148 cm3. Use the formula Volume of a cube V = s where S= (2.8148) ^1/3 to work out the length. This will give you Length (s) of a cube = 1.4119 cm.
V = M/D so V = 7.6/2.70 or V = 2.814(repeating bar over 814) so lets just estimate ~2.81 Now take ~2.81 (~ means about or approximately) and find the cubed root of it since you're trying to find one edge. That would make one edge ~ 1.41 centimeters since volume is cubic centimeters, so your final answer is ~1.41 cm.