Answer:
a) Yes agree
and b) (I'm not too sure about this one but I think..) By CPCT that is Corresponding parts of congruent triangles are equal/congruent theorem.
Explanation:
I'll use this " [ " symbol for angle.
a) yes agreed ;
AB = CB (B is the midpoint of AC)
[ ABD = [ CBD = 90°
BD = BD (common side)
--> therefore by SAS criteria triangle ABD is congruent to triangle CBD.
and by CPCT .... AD = CD .
b) AD = CD (proven by CPCT)
...answer for b you're likely suppose to write it that way. so yeah hope this helps.