Answer
$$4.0\ g /cm^3$$
Work Step by Step
We have
$$
\frac{140\ g }{ (3.2 \ cm)(1.7 \ cm)(6.4 \ cm)}=4.021\ g /cm^3
.$$
Now, round this product to two significant digits, which is the accuracy of the least accurate measurement. That is,
$$4.021\ g /cm^3=4.0\ g /cm^3$$
