Population standard deviation for 75 80 90 95 75 85 55 80 90 95

So in the calculator you go to STAT, EDIT then put the numbers in L1 and then hit STAT, CALC, #1( 1-vars stat) and go to the one under Sx. and that is your population standard deviation. the standard deviation would be Sx but since your are finding the population standard deviation, it would be the one under Sx. The answer is 11.44552314. but rounded, it is 11.45. 
Hope this helps!!

To find f(-20), first figure out which piece x = -20 fits with.

Since -20 < -12, x = -20 first in the domain used by the third piece.

For f(-20), treat this function as if it was just f(x) = 3x-7.

f(-20) = 3(-20) -7

         = -60 - 7

         = -67