Find all unit vectors that are perpendicular to both a = (1, 2, 1) and b = (1, -3, 2).

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I tried having a third vector c = (x, y, z) and for it to be perpendicular, the dot product of a and c must equal 0 and the dot product of b and c must equal 0. But this only gave me two equations with three unknowns, so I am unsure where to go from here.

Thanks!