I would hope that you at least know that youcan'tmultiply three vectors like that!

The orthogonal complement of a three dimensional subspace is aonedimesional subspace so you want to find a single vector (a, b, c, d) such that (a, b, c d).(1, 1, 3, 2)= a+ b+ 3c+ d= 0, (a, b, c, d).(1, 1, -2, -3)= a+ b- 2c- 3d= 0, and (a, b, c, d)(2, 1, -3, -1)= 2a+ b- 3c- d= 0. The orthogonal complement is the suspace of all multiples of that vector.