平泳ぎは出来るが、背泳ぎは出来ない。

、。

I can do the breaststroke but not the backstroke.

Literal

Breaststroke [topic-は] can-do but, backstroke [topic-は] cannot.

平泳ぎ (breaststroke) and 背泳ぎ (backstroke) both use 泳ぎ (swimming) with descriptive prefixes: 平 (flat/level) and 背 (back). The は...が...は pattern creates a contrast: 'this one I can, but that one I can't.' 出来る is written in kana here instead of the more common できる.