∫∠MAN = 90？ , AB divided equally ∠AMN

∴∠NAB= 1/2×90？ =45?

∵BC⊥AB

∴∠ACB=90？ -∠NAB=45

(2) Make BF⊥AM,BG⊥AN, and the vertical feet are F and G respectively.

∵AC=6,∴BF=BG=3

AD=t, AE=2t, EC=6-2t.

∵S⊿ABD:S⊿BEC=2:3

∴ 1/2 t×3: 1/2(6-2t)3 = 2:3

∴t= 12/7

(3) there exists a certain time t that makes ⊿ABD≌⊿CBE,

∠∠ECB =∠DAB = 45？ ，CB=AB

∴CE=AD

∴6-2t=t

∴t=2