Answer
The mass of the smaller star is $1.51\times10^{30}kg$
Work Step by Step
We assume the center of mass has position $x_{cm}=0$, and the center of the larger star has position $x_l=2.08\times10^{11}m$
From here, we can find the center of the smaller star $x_s=(2.08\times10^{11}m)-(7.07\times10^{11}m)=-5.09\times10^{11}m$
Therefore, $$x_{cm}=\frac{m_lx_l+m_sx_s}{m_l+m_s}=0$$ $$m_lx_l+m_sx_s=0$$ $$m_s=\frac{m_lx_l}{-x_s}=1.51\times10^{30}kg$$
