I think this is a parallelogram prism?

This is the components given:

vectors:

a=(2, -5, -1)

b=(3,0,1)

c=(3,-1,-1)

So, I thought if this was a parallel prism, find a cross-product b, get the magnitude (area of parallelogram) and then multiply by magnitude of the third vector (depth?). But I get the wrong answer. I'm getting 71, the answer is 29. Any help?