DB is a median (given). It goes from vertex point D to midpoint B.
The point B is the midpoint of segment AC. So B cuts AC into two equal halves: AB and AC
Meaning AB = AC
AC = AB+BC ... segment addition postulate
AC = AB+AB ... substitution; replace BC with AB (valid because AB = AC)
AC = 2*AB
2*AB = AC
2*AB = 50 ... substitution; replace AC with 50
2*AB/2 = 50/2 ... divide both sides by 2
AB = 25 which is the answer
