how does the square root of 20 equal plus or minus 2 the square root of 5?

how was that answer gotten?
 
√20 = + - 2√5

when we have a positive number under the square-root, we can have a positive or negative number.  this is because a positive times a positive is positive.  also, a negative times a negative is positive.  in addition, when we simplify radicals, we break them down by its factors.
 
 
±√(20) =
 
±√(4)√(5) =
 
±2√(5) =
 
±2√(5)