A mad puzzle enthusiast locks you inside a room with only one door. Next to the door is a standard numeric keypad. There is no way to escape except through the door. You find a piece of paper next to the keypad:
$181 \Rightarrow E \unicode{12288} 3$
$3 \unicode{12288} -E \Rightarrow \unicode{xFF1F}$
You turn the piece of paper over and see two more scribbles.
$E = 1$
The answer is not in sight.
What do you type in the keyboard to escape, and how did you figure it out?
Hint 1:
You notice that the puzzle enthusiast had one more equation but erased it. Following the indentations on the paper, you find out that it said: $3 \unicode{x005F} 1 \Rightarrow 1A3$
Big Hint 2:
The numbers on the keypad are all in seven-segment display. You wonder if that means anything...
Since this is my first puzzle, any feedback would be appreciated.