But that's so easy! I should've thought of it for you.
6 seconds sound clip from the The Wizard of Oz (1939) movie soundboard.
You can hear this line at 01:37:12 in the Blu-ray version of the movie.
Quote context
[...]
- Then why didn't you tell her before?
- Because she wouldn't have believed me. She had to learn it for herself.
- What have you learned, Dorothy?
- Well. I think that it... that it wasn't enough just to want to see Uncle Henry and Auntie Em.
- But that's so easy! I should've thought of it for you.
- I should've felt it in my heart.
- No. She had to find it out for herself.
- Now those magic slippers will take you home in two seconds.
- Now?
[...]