Answer
$m_2=1.4kg$
Work Step by Step
When the first block is hung, $$m_1g=F_x=kx_1$$ $$k=\frac{m_1g}{x_1}$$
When 2 blocks are hung, $$(m_1+m_2)g=F_x=kx_2$$ $$k=\frac{(m_1+m_2)g}{x_2}$$
Therefore, $$\frac{m_1g}{x_1}=\frac{(m_1+m_2)g}{x_2}$$ $$\frac{m_1+m_2}{m_1}=\frac{x_2}{x_1}=3$$ $$1+\frac{m_2}{m_1}=3$$ $$m_2=2m_1$$We know $m_1=0.7kg$, so $m_2=1.4kg$
