Grade 8 Geometry
posted by Selena .
to calculate volume you use the formula
V= area of b.s.(base area) x h(height)
 How can I tell what face is the "base shape", especially with a triangular prism?

In a triangular prism V=(1/3)x(Base Area)x(Height)
To find Base Area in the case of triangular prism we use the formula for area of a triangle that is A=(1/2)x(L+W) 
Joe, what you stated is the volume of a pyramid.
The volume of a triangular prism = base area x height
http://www.google.ca/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/2/21/Triangular_prism.png&imgrefurl=http://commons.wikimedia.org/wiki/File:Triangular_prism.png&h=1000&w=1000&sz=147&tbnid=3nAf5nE8vVoM8M:&tbnh=149&tbnw=149&prev=/images%3Fq%3Dtriangular%2Bprism&hl=en&usg=__i8aqcfEydQvVJHrJFpcgp_Du9bQ=&ei=de7uS5HGsT48Abat9T9Cg&sa=X&oi=image_result&resnum=1&ct=image&ved=0CBgQ9QEwAA