sorry for my late reply.
finally got something to work.
here's how i kinda solved the issue:
1. enter grub recovery mode, and choose root shell
2. remount the filesystem so that can read and write using command "mount -o remount,rw /"
3. edit the grub config using "sudo nano /etc/default/grub"
4. modify the line GRUB_CMDLINE_LINUX_DEFAULT="quite splash" to GRUB_CMDLINE_LINUX_DEFAULT="quite nomodeset"
5. uncomment the GRUB_GFXMODE line
6. save and write the changes using command Ctrl o + Ctrl x
7. rebuild the grub config using command "sudo update-grub"
8. reboot my laptop using command "sudo reboot"
after that, i can login to backbox on tty1, and then type command startx to start the graphical mode.
anyway, thanks Zerof for the help!