Evan H. answered • 11/15/14

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Another method, although not quite as simple as Francisco's solution, is pretty elegant and more linear algebra-y(if this is for a linear class) is to take the determinant of the three vectors.

If the determinant is 0, they are linearly dependent and coplanar:

det (4 -1 h)

(1 2 -1)

(-2 -1 1)

= 4 ( 2 - 1) + 1(1 - 2) + h(-1 + 4) = 0

4 - 1 + 3h = 0

h = - 1

Same solution as you can see.