Here's the question you clicked on:
DrHungrysaur
In Part A, Section 6: Volumes and Determinants in Space, Determinants 1, Example 1.1 the solution seems to not correspond to the problem at all. The 2x2 determinant gives almost trivially 1 as an answer... what is going on here, an editing mistake?
I guess you're right, the solution is 1, the result is obviously about another matrix.