19 √21⋅√14/√6

√21⋅√14/√6 = √(21 * 14) / √6 = √(3 * 7 * 2 * 7) / √6 = √(3 * 2 * 7²) / √6 = 7 * √(3 * 2) / √6 = 7 * √6 / √6 = 7