ok, so one way is, like this divide it by 100 and change to square cm
the way to see why this works is like this
first, 1mm=1/10cm so if the prism was flattened and cut and moved into a rectangle let's say legnth was 10mm andn width was 5mm convert each to cm legnth is 1cm and width is 0.5 0.5cm^2 is the area
