Answer:

There are no beginning things to be done, so that blank can be left empty. The ending things is just writing out the word "Bye":

Completing the Loop Body Blank

So with two of the blanks completed, the program looks like:

PRINT "Enter first number"       
INPUT NUMBER                 
'
DO WHILE ... number is not a sentinel value ...
 
  . . . do something or other . . .    
 
  PRINT "Enter the next number."       
  INPUT NUMBER                 
LOOP
PRINT "Bye"
END

Remember what we want the output to look like:

Enter the first number
? 3
Square root is:  1.7320508
Enter the next number
? 4
Square root is:  2
Enter the next number
? -10
Bye

QUESTION 17:

Decide how to complete the blank:

. . . . do something or other  . . .

Remember that in QBasic SQR( NUMBER ) is the square root of NUMBER.