Apply dimensional analysis to complete the following unit conversions and report your answers in scientific notation. Show detailed work. Be sure to round each answer to the correct number of significant figures.

a$)3,500$ micrometers to kilometers

b$)\frac{1}{3}$ pound to milligrams $)$

$($Hint: The fraction, $\frac{1}{3},$ is an exact number. When setting up dimensional analysis, be sure to write it as a fraction. What number is in the numerator? in denominator? How about the unit pound? in numerator or denominator?$)$