I would hope that you at least know that you can't multiply three vectors like that!
The orthogonal complement of a three dimensional subspace is a one dimesional 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.
You have, as I said, three equations in four variables.
a+ b+ 3c+ d= 0, a+ b- 2c- 3d= 0, and 2a+ b- 3c- d= 0
Find all a, b, c, and d that satisfy those equations.
Personally, I prefer to solve systems of equations as I learned way back in high school rather than using matrices. For example, if you subtract the second equation from the first, you eliminate both a and b: 5c+ 4d= 0. If you subtract the second equation from the third you eliminate b: a- c+2d= 0. From 5c+ 4d= 0, c= -(4/5)d. Then a- c+ 2d= a+(4/5)d+ 2d= a+ 14/5d= 0 so a= -(14/5)d. The first equation becomes a+ b+ 3c+ d= -(14/5)d+ b- (4/5)+ d= b- (13/5)d= 0 so b= (13/5)d.
(a, b, c, d)= (-(14/5)d, (13/5)d, -(4/5)d, d) where d can be any number. If you don't like fractions, take d= 5 so your vector is (-14, 13, -4, 5) and the "orthogonal complement" is spanned by that vector.