Latex starts the new line at different points

We need to minimize: $z=\sum_{i=1}^{15}x_i$ \\\\

Subject to:\\

$x_{door,1}+x_{door,4}+x_{room,A \, \geq \, 2}$ (Room $A$) \\

$x_{door,3}+x_{door,8}$

So i have this in latex. The problem is that latex starts the line after \ at different points so the sentences are not perfectly aligned vertically. After the first two \ latex starts left from where it started the 'we need' sentence and after ''subject to:\'' it starts right from where it started the 'subject to' sentence?? Please help, thanks in advance.