Perpendicular Vectors find x

Question

My Higher maths question booklet has this question and I am rather stuck with it. I have done one similar before I just cannot remember and have no idea now.

      Two vectors **a** and **b** are perpendicular.
      **a** = i – 2j + xk and **b** = xi + 3j + xk
      Find and verify any possible values of x.

Answer 1

Hint: $\vec a \perp \vec b \Leftrightarrow \vec a \cdot \vec b = a_x b_x + a_y b_y + a_z c_z = 0$

Answer 2

compute the dot-product:$$(i-2j+xk)\cdot (xi+3j+xk)=x-6+x^2$$