Question

what is the height of a solid with a volume of 120 m3 and a base of 30 m2

Answer 1

volume of solid = Area of the base * height
therefore:
120 = 30 * h
h = 4 m

so there,, :)

Answer 2

You do not say if it is a cylinder (sides straight up and down) or some other shape like a cone or a half sphere or whatever.
If the sides go straight up then
30 h = 120
h = 4 meters
If it is a cone or pyramid (straight sloping sides)
(1/3) (30) h = 120
h = 12 meters