Answer
$A = 1.50\times 10^{-4}~mm^2$
Work Step by Step
First we can express the area in units of $m^2$. Note that $1 ~\mu m^2 = 1\times 10^{-12}~m^2$
$A = 150~\mu m^2\times \frac{1\times 10^{-12}~m^2}{1~\mu m^2}$
$A = 150\times 10^{-12}~m^2$
We can express the area in units of $mm^2$. Note that $1 ~mm^2 = 1\times 10^{-6}~m^2$
$A = (150\times 10^{-12}~m^2) \times \frac{1~mm^2}{10^{-6}~m^2}$
$A= 150\times 10^{-6}~mm^2$
$A = 1.50\times 10^{-4}~mm^2$
