Given
F = 55 N to ring the bell
P = 250 W
t = 0.85 s
Procedure
First let's calculate the force for 250W of power.
[tex]\begin{gathered} P=\frac{Fd}{t} \\ F=\frac{Pt}{d} \\ F=\frac{250W\cdot0.85s}{7m} \\ F=30.35\text{ N} \end{gathered}[/tex]With 250W it is not possible to reach the bell to ring it.
Now let's calculate the value for 500W
[tex]\begin{gathered} F=\frac{500W\cdot\text{0}.85s}{7m} \\ F=60.71\text{ N} \end{gathered}[/tex]Force is doubled when power is doubled