Well, lets break this down.
Volume = area of base * height
we know that the volume is 100 and the height is 30 so we can work backwards
100 = b * 30
now divide both sides by 30
(100/30) = b * (30/30)
the height cancels itself out and we are left with...
base = 3.33
therefore your answer is 3.33 cm^2
