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const array1 = [12, 13, 14, 15, 16, 17, 18, 19, 20];
const array2 = [17, 18];
const length = 3;
const indexOf17FromArray1 = array1.indexOf(array2[0])
console.log(indexOf17FromArray1);
console.log(array1.slice(indexOf17FromArray1 - length, indexOf17FromArray1));
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