ASUS X551CA

For this model, on the official site there is drivers only for Windows 8 and 8.1.

HERE IS THE LINK

Here we have the drivers for Windows 7.  All the drivers are packed in a rar file.
You can download this file from this link.

ASUS X551CA DRIVERS

There is one driver missing from the package. That is Card Reader Driver that you can download from this LINK.