The prism has a volume of 70 mm3.
find the volume of a scaled image with a scale factor of 3.
mm3

So if it is scaled down then it is divided by 3
if it is scaled up then is it multiplied by 3

so if it is scaled up then
5 times 3=15
7 times 3=21
3 times 3=9
15 times 21 times 9=2835 mm^3

if it is scaled down then
5/3 times 7/3 times 3/3=5/3 times 7/3 times 1=5/3 times 7/3=35/9=aprox=3.8888889.mm^3