JSFiddle - React, Tailwind, and code Playground

JavaScript

var obj = {
  array1: [1, 2, 3, 4, 5],
  array2: [6, 7, 8, 9, 10],
  array3: [51, 52, 53, 54],
  array4: [51, 52, 53, 54, 7]
}

function finder(obj, test) {
  var keys = Object.keys(obj);
  var out = [];
  for (var i = 0; i < keys.length; i++) {
    var key = keys[i];
    if (obj[key].indexOf(test) > -1) {
      out.push(key.match(/\d+/)[0]);
    }
  }
  return out;
}

console.log(finder(obj, 7));