Answer:
[tex]\large\boxed{39^o}[/tex]
Step-by-step explanation:
[tex]m\angle ABC=90^o\\\\m\angle ABC=m\angle ABD+m\angle DBC\\\\m\angle DBC=51^o\\\\\text{Substitute:}\\\\m\angle ABD+51^o=90^o\qquad\text{subtract }\ 51^o\ \text{from both sides}\\\\m\angle ABD=39^o[/tex]
