What namespace for IActionResult

I know it's outdated, but we have an existing Web Api Project using .Net Framework 4.7.2. We are not in a position to upgrade the whole project, but I need to introduce a new method in the form:

public async Task<IActionResult> myMethod(string requestUrl)
{
   ......
   Return Content(result, "application/json");
}

I dont have a reference to the library that supports IActionResult, can someone tell me what the reference should be for 4.7.2