VTMO 2016 — Grade 9 — semifinal Question 7

$A \left(0 , a\right) , D \left(0 , - 1\right) , B \left(2 , - 1\right) , C \left(3 , 2\right) , O \left(0 , 0\right)$. Triangles $ADB$ and $CBO$ have equal areas. Find a as shown.