Hi Elisa,
use a multi-line equation environment of amsmath, such as align, split or multline. For example:
Code: Select all
\begin{multline*}
P(t,s,r)= \exp\bigg[\frac{1}{\alpha} (1- \exp
\left(- \alpha (s-t)\right) (R(\infty)- r) -(s-t) R(\infty)\\
-\frac{\rho ^2}{4 \alpha^3}(1- \exp \left(-
\alpha (s-t)\right ))^2 \bigg]
\end{multline*}
or
Code: Select all
\begin{equation*}
\begin{split}
P(t,s,r)&= \exp\bigg[\frac{1}{\alpha} (1- \exp
\left(- \alpha (s-t)\right) (R(\infty)- r) -(s-t) R(\infty)\\
&\quad- \frac{\rho ^2}{4 \alpha^3}(1- \exp \left(-
\alpha (s-t)\right ))^2 \bigg]
\end{split}
\end{equation*}
Note, matching \left and \right delimiters need to be on the same line, that's why I used \bigg[ and \bigg] instead, as the expressions spans over two lines.
Stefan