そんなことは朝飯前(あさめしまえ)だ。
朝飯前(あさめしまえ) means “something you can do even before eating breakfast,” i.e. something very easy. Something similar to “a cinch” or “a piece of cake” in English.
きのうのしゅくだいなんて朝飯前(あさめしまえ)だった。
Yesterday’s home work was so easy!あんなやつをまかすのは朝飯前(あさめしまえ)だ。
Beating him will be a cinch.漢字(かんじ)の練習(れんしゅう)なんて朝飯前(あさめしまえ)さ。
Kanji practice is a piece of cake!
朝飯(あさめし) is breakfast, which can also be called 朝(あさ)ごはん or 朝食(ちょうしょく) but in this expression, you cannot substitute あさめし with あさごはん or ちょうしょく.