\[ ... \] method of setting a display can handle only one line.
amsmath package provides a number of environments that set multiple lines in various arrangements. see the manual for details. since you apparently want a display with no number, you need to use a starred environment; @huitseeker has already suggested
e \leftarrow bla bla \\
r \leftarrow x + y
and that's a good answer, but he first suggested
eqnarray, which is generally bad for mathematical documents. see this article by lars madsen for the reasons.
although no alignment is explicitly requested, it's implicit in the example shown that it is actually wanted. without specifying an alignment point (with
&) the lines will be aligned on the right, so if one is noticeably longer than the other, the result will not be attractive. alignment of relations is the usual practice, so inserting
\leftarrow in both lines is probably the best approach.
if alignment is really not wanted, and both lines should be centered, the
gather environment is more appropriate.