I'm looking for a regular expression for the language ![$w\in {a,b}^n$](https://i.stack.imgur.com/f0HhV.png) with the exact number of ***k a***'s in it.

I'm pretty much stuck at this. For a various length the solution would be easy with ![${{b}^*a{b}^*}^k$](https://i.stack.imgur.com/LIiTa.png).

Does anybody have any advice on how I can achieve such an regex?