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## Homework Statement

Two charges Qa = 3 µC and Qb = -3 µC are placed on the x-axis with a separation of a = 21 cm.

(a) Find the net electric field at point P, a distance d = 13 cm to the left of charge Qa.

This is no prob = -1364069.13N

(b) Find the force on Qb due to Qa .

No prob again = -1.837

The charges Qa and Qb are now attached to the ends of a spring whose unstretched length is s0 = 21 cm. With the charges attached, the spring compresses to an equilibrium length s1 = 9 cm.

(c) Calculate the spring constant ks of the spring.

this is what i cant figure out?

## Homework Equations

## The Attempt at a Solution

heres are the steps i did (its wrong though)

1. Spring constant is Fsp = -k delta s

2. i already know the force that the 2 charges excert on each other (= 1.837N)

3. delta s would be 21cm - 9cm = 12cm = .12m

4. then i just plugged in:

1.837 = -k(.12m)

k = -15.308

this is wrong.

please help