Answer
$$\approx 8.52kg$$
Work Step by Step
$$\dfrac {F_{1}}{A_{i}}=\dfrac {F_{0}}{18A_{i}}\Rightarrow F_{1}=\dfrac {F_{0}}{18}=\dfrac {k\Delta x}{18}=\dfrac {3\times 10^{4}\times 5\times 10^{-2}}{18}\approx 83.33N\Rightarrow m=\dfrac {83.73}{9.78}\approx 8.52kg$$
