You can use Vroot, A Windows utility that makes rooting a one-click affair, to root your LG MS769 Optimus L9.
Step 2: Enable USB debugging mode on your phone.
On Android 4.0 or 4.1, tap Settings, Developer Options, then tick the box for “USB debugging.” (You may need to switch “Developer options” to On before you can do so.)
On Android 4.2 and later, tap Settings, About Phone, Developer Options, and then tick USB debugging.” Then tap OK to approve the setting change.
Step 3: Run Vroot on your PC.
Step 4: Once vRoot Application is launched, connect your Android device to the computer.
Step 5: Once Your Optimus L9 is connected successfully to the computer, vRoot will automatically detect your Smartphone.
Step 6: Now, Click on the “Root” button to start the rooting process.
Step 7: vRoot will automatically reboot your Smartphone, and Now Your Phone is rooted!