TeX source:

\begin{split}\Pr\{\text{even}\} &= \text{tr}\{\mathbf{A}_{\text{even}}\rho\} = \text{tr}\{(|00\rangle\langle00| + |11\rangle\langle11|)\rho\} \\ &= \text{tr}\{|00\rangle\langle00|\rho\} + \text{tr}\{|11\rangle\langle11|\rho\} = \text{tr}\{\langle00|\rho|00\rangle\} + \text{tr}\{\langle11|\rho|11\rangle\} \\ &= \langle00|\rho|00\rangle + \langle11|\rho|11\rangle,\end{split}