In order to simplify the following expressions you use the one of the square root properties:
√a√b = √(ab)
5)
√(5n^3)√(20n) = √((5n^3)(20n)) = √(100n^4) = 10n^2
6)
√(8p)√(5p^3) = √((8p)(5p^3)) = √(40p^4) √(4*10p^4) = 2p^2√10
7)
3√(15n^2)√(5n^2) = 3√((15n^2)(5n^2)) = 3√(75n^4) = 3√(25*3n^4) = 15n^2√3
8)
√(3p^3)√(3p) = √((3p^3)(3p)) = √9p^4 = 3p^2