It is "knitted her brow", "furrowed her brow", or "knitted her brow in a furrow". I believe you want furrow instead of "frow".

I wouldn't have got this without Roxanne (you don't have to turn on the red light - "The Police" Sting's hard to understand, he's from Newcastle like me :) ) saying, "furrow". She's right though. A furrow is a trench or deep wrinkle in the skin. To knit your brow, means to frown/look angry or displeased. And you'd use, "a frown". Why people say, "knit" I've no idea, knitting is how you make jumpers out of wool. Unless it means to contort your expression to be tightly pressed together, like the strands of wool.