Im stuck on the following question

Find the value of for which (1, A, 3) and (A, 2, 1) are orthogonal. Using that

value of A, find a third vector orthogonal to both of them.

iv done the cross product on them and got the following result (A+6)i - (1-3A)j + (-2-$\displaystyle A^2$)k but cant see anyway to actually determine A.

Thanks for any help