The volume is the product of length, width, and height. This is a formula you should commit to memory.
.. V = l*w*h
The base is square, so the length and height are both 25 mm. The length is given in cm, so you need to convert units so all the units match*. You know 1 cm = 10 mm, so the length is 120 mm. Or, if you convert the units of the base, you get the base is 2.5 cm square.
The numbers come out smaller and perhaps more understandable if they are given in cm, so let's do that.
.. V = (12 cm)*(2.5 cm)*(2.5 cm)
.. V = 75 cm^3
