Answer
$21\;cm^2=2100\; mm^2$.
Work Step by Step
Conversion factor is
$=\frac{100\; mm^2}{1\; cm^2}$
The given unit of measure is
$=21\;cm^2$
Multiply by the conversion factor.
$=21\;cm^2\cdot \frac{100\; mm^2}{1\; cm^2}$
Simplify.
$=2100\; mm^2$.
