I need help for #1 (an explanation would be great too).

±6i√2

Step-by-step explanation:

Recall that the square root of a negative number is imaginary, which is shown by the use of "i."

Examples:  √4 = ±2; √4 = i√4 = ±i2

Then √72 = ±√36√2 = ±6√2, and

      √(-72) = ± i√36√2 = ±6i√2

Note that this is identical to the result shown in the problem:  √(36·2·(-1) )