/**
 * You can edit, run, and share this code.
 * play.kotlinlang.org
 */

fun main() {
    val a = listOf(2,3,9,2,5,1,3,7,10)
    val b = listOf(2,1,3,4,3,10,6,6,1,7,10,10,10)

    val result = mutableListOf<Int>()

    a.forEach { item ->
        val count = b.count { item == it }
        if (!isPrime(count)) {
            result.add(item)
        }
    }

    println(result)
}

fun isPrime(num: Int): Boolean {
    if (num < 2) return false

    for (i in 2..num / 2) {
        if (num % i == 0) {
            return false
        }
    }

    return true
}

Time complexity:
If I'm understanding this correctly, the time complexity will be O (n * (m/2))
Where
n is the length of list a
m currently evaluated item from the list a

Aww it's been a while since I did this O notations thing