$5\mathrm{.}11(10$ pts$)$ The total line length of the dislocations visible in a $4$ cm by $4$ cm TEM photograph of a

metal foil, taken at a magnification of $20,000$x is measured as $400$ cm $.$ The foil specimen imaged by

this picture had a thickness of $300$ nm $.$ Determine the dislocation density in the specimen.