s.substring(s.indexOf(x)) replaceable with s.substringAfter(x).
Using a corresponding functions makes your code simpler.
The quick-fix replaces a substring call with substringAfter.
Example:
fun foo(s: String) {
s.substring(s.indexOf('x'))
}
After the quick-fix is applied:
fun foo(s: String) {
s.substringAfter('x')
}