Simplifying Radical Expressions

I've been working on these problems for a little over an hour and i can't for the life of me figure out how to do them.  Can someone walk me through these? Hopefully I'll start to get the hang of it.
1.)  (-2 √15)(4 √21)

2.)   3∛128+5∛16

3.)  √(50x4)
  

These depend on the directions instructing to keep in simplest radical form
or take to a decimal. These answers will be in simplest radical form.

1.) (-2√15)(4√21)
= -8√315 {multiply}
= -8√9 √35 {depending on the directions whether to take to a decimal or not}
= -8(3)√35 {square root o{f 9 is 3}
= -24√35 {multiply}

2.) 3∛128+5∛16
==3∛64 ∛2) + 5∛8 ∛2 {break down the cube roots again directions dependent}
= 3(4)∛2 {cube root of 64 is 4}
= 12∛2 {multiply 3 by 4}

3.) √50x4
=√25x4√2 {group the perfect squares together under one square root sign}
= 5x2√2 {take the square root of 25x4}  

- Algebra House