4.99
Step-by-step explanation:
The mnemonic SOH CAH TOA reminds you that the relationship between adjacent and opposite sides of an angle in a right triangle is ...
 Tan = Opposite/Adjacent
In you diagram, this is ...
 tan(31°) = AB/BD
Multiplying by BD/tan(31°) gives ...
 BD = 3 units/tan(31°) ≈ 4.99 units