If you have a square number, you must open it. The mistake of your first question is that it is not clean. For the convenience of observation, we usually don't multiply the product, but divide it into completely squared numbers, which is easy to distinguish.

( 1) √24×√27=√24×27=√4×6×3×9=√4×3×2×3×9=√4×9×9×2= 18√2

(2) √6×(-√ 15)=-√6× 15=-√3×2×3×5=-3√ 10

(3) √ 18×√20×√75=√9×2×4×5×25×3=√9×4×25×2×5×3=30√30

The above three numbers are not completely squared.

(4)√3×3×4×4×4×5=√9× 16×4×5=24√5

Only 5 is not a complete square number.