וַיַּ֥עַשׂ מֹשֶׁ֖ה וְאַהֲרֹ֑ן כַּאֲשֶׁ֨ר צִוָּ֧ה יְהוָ֛ה אֹתָ֖ם כֵּ֥ן עָשֽׂוּ׃
I uczynił tak Mojżesz i Ahron; jako przykazał im Wiekuisty tak uczynili.
Or HaChaim
ויעשו כן משה ואהרן, Moses and Aaron did so precisely, etc. This means that when Moses and Aaron smote the river the result was that G'd's will was done and the result was the same as if G'd Himself had struck the river. Please read what I have already written on verse six concerning such expressions in the Torah, as well as what I shall be writing on the verse following.