The volume of any prism is equal to its base times its height. V = Bh
A figure such as a pyramid or cone has 1/3 of the volume of the prism which contains it. V = Bh/3
In the case of our pyramid here...
B = 15√3 cm²
h = 4 cm
Plug these into our formula...
V = Bh/3 = (15√3)(4)/3 = (60√3)/3 = 20√3 cm³