Addition of vector easy question

If resultant of vector $\overrightarrow{\mathrm{a}} \mathrm{and} \overrightarrow{\mathrm{b}}$ is ⊥ $\overrightarrow{\mathrm{a}}$ then find out angle between $\overrightarrow{\mathrm{a}} \mathrm{and} \overrightarrow{\mathrm{b}}$.
$$\begin{gathered} \mathrm{\alpha }=90° \\ 90=\frac{\mathrm{Bsin\theta }}{\mathrm{A}+\mathrm{Bcos\theta }} \end{gathered}$$
$\mathrm{B}·\mathrm{B}-9.$