What equation/thing is happening to this number to get this solution? my book is just telling me "use a calculator" but it doesn't explain it any further. !

Use the square root function on your calculator

Step-by-step explanation:

Look at the third line down, where is says that 936 = AD^2.  If you hit the 2nd button on your calculator then the x^2 button, you will get the square root function.  After you hit those buttons in that order, you'll get the opportunity to find the square root of 936 by entering it in and hitting "enter" on your calculator.  The square root of 936 is 30.59417