Valence electrons in the element A are 3 and that in element B are...

Valence electrons in $A = 3$

Valence electron in $B = 6$

Thus, $A$ is electropositive and $B$ is electronegative, $A$ can lose three electrons and $B$ can gain two electrons to attain stable configuration.

Hence, $A$ exist as $A^{3 +}$ and $B$ as $B^{2 -}$

$∴$ Compound formed is ${A}_{2}{B}_{3}$