It was a piece of cake.

It was a piece of cake is the response of "you really aced that one" as they are both away to tell each other that they're ace.

"You really aced that one." He said as he looked at the drawing of the dragon.
"It was a piece of cake." she replied
His eye widen then hers. FINALLY.
"YOU TOO?" she hadn't meant to scream it, but Holy Fuck . He nodded, torn between shushing you and screaming as well.
"Wanna get cake sometime or something?"

Piece of cake walk in the park

Something that's extremely easy. A combination of the phrases "Piece of cake", "Cake walk", & "Walk in the park", all of which mean something that's very easy. A piece of cake is a piece of cake & a cake walk is walking with a cake, so a piece of cake walk would be walking with a piece of cake. A walk in the park is walking in the park, so a piece of cake walk in the park would be walking in the park with a piece of cake.

I wasn't fazed by Fukouna Shoujo 03. It was a piece of cake walk in the park.

A piece of cake

1. Very easy to do.
2. Maybe it’s really a piece of delicious cake.

Preparing a simple breakfast like scrambled eggs? It’s a piece of cake!