Simplify, add, and subtract radicals

Match each of the following to its correct simplified form.

Simplify 
3√6 - 4√6

• -√6
• √6
• -1
• Already simplified 

Simplify 
-11√21 - 11√21

• 0
• -22√21
• √21
• Already simplified 

Simplify 
3√8 + 3√2

• 9√2
• 6√10
• 5√2
• Already simplified 

Simplify 
3√18 - 2√2

• 7√2
• √16
• √2
• Already simplified 

Simplify 
-3√20 - √5

• -4√5
• -4√15
• -7√5
• Already simplified