To find the force that Sarah expends, we can use the formula for work:
\[ \text{Work} = \text{Force} \times \text{Distance} \]
where:
- Work is the work done (in Joules),
- Force is the force applied (in Newtons),
- Distance is the distance over which the force is applied (in meters).
We can rearrange the formula to solve for Force:
\[ \text{Force} = \frac{\text{Work}}{\text{Distance}} \]
Substituting the values we have:
\[ \text{Force} = \frac{84 , \text{J}}{3 , \text{m}} = 28 , \text{N} \]
Therefore, the amount of force Sarah expends is 28 N.