documentclass{article}
usepackage{amsmath}
DeclareMathOperator{E}{E}
begin{document}
begin{multline*}
P_t=
frac{1}{2}Bigl(m_t+v_t-u_t
+(frac{1-zeta }{2})(frac{E_{t-1}m_t+frac{1-zeta }{2zeta }E_{t-
2}m_t+frac{zeta (1-zeta )}{2(1+zeta )}E_{t-2}P_{t-1}+frac{zeta }
{2}P_{t-1}+frac{zeta }{1+zeta }P_{t-2}}{(1+zeta )}))bigr\
+frac{(1-zeta )}{4}Bigl(frac{E_{t-2}m_t+(frac{zeta }{2})(E_{t-2}P_{t-
1}+P_{t-2})}{(1+zeta )})Bigr) \
+frac{zeta }{4}(Bigl((P_{t-1}+P_{t-2})Bigr)
end{multline*}
end{document}