Line equation : general understanding (Need ASAP help , exam tomorow)

Hello everyone ,

This is my first time here and because i dont have a lot of time left due of my examination tomorrow i will not spend much time on the grammar.

The point A has coordinates (7,7) and the line L has equation x+3y=8

(i) Find the equation of the line that passes through A and is perpendicular to L

I cant understand generaly what am i supposed to do here , plz provide me step by step with no "wordy" words , as simple as possible :p