Hooke's law describes a certain light spring of unstressed length 36.0 cm. When one end is attached to the top of a door frame and a 6.40 kg object is hung from the other end, the length of the spring is 44.00 cm.

Question

Hooke's law describes a certain light spring of unstressed length 36.0 cm. When one end is attached to the top of a door frame and a 6.40 kg object is hung from the other end, the length of the spring is 44.00 cm.
(a) Find its spring constant
I found the correct answer to be k=784 N/m

(b) The load and the spring are taken down. Two people pull in opposite directions on the ends of the spring, each with a force of 170 N. Find the length of the spring in this situation.

I thought the answer was 0.7937m but this is incorrect. I have no other idea on how to solve for part b. 
Thank You!

drwls · Answer

The length of the spring under tension is the amount of stretching PLUS the original unstressed length. The stretch amount is F/k = 170/784 = 0.217 m = 21.7 cm Add 36.0 cm to that.