The new angular frequency of the system will be:

Angular frequency $\omega =\sqrt{\frac{k}{m}}$

when new block gets attached with previous one 

$m_f=m_1+m_2$

$m_f=13+12=25kg$

${\omega }_f=\sqrt{\frac{k}{m_f}}$

${\omega }_f=\sqrt{\frac{2500}{25}}$

${\omega }_f=\sqrt{100}$

${\omega }_f=10rad/s$