We're NOT talking about a pyramid or tetrahedron, right? This is a right-prism with triangular cross section, possibly standing on a triangular end (but that's not particularly necessary).
Area of Triangle is (1/2)*base*height. We are given height = 4 (I'll let you worry about the units), so we have: Area = 2*base
Volume of such a prism = BaseArea * Height. We are given Volume = 16 and Height = 8, so we have 16 = BaseArea * 8
Thus, BaseArea = 2.
You do the rest.
Note: It is quite unclear how "height" and "altitude" are arranged. I had to guess, picking "altitude = altitude of base triangle".