I see the word "when..." kind of fading out at the end of the first line. Whatever comes after it may be important.
If you're just supposed to copy the expression into the box, then the problem is that you left the 'e' out of it.
I'm guessing that you're supposed to enter whatever the expression becomes when either N₀ or ' t ' has some special value that's in the first line.
Just taking a wild guess here . . . . .
If it's "Enter the expression ..... , when t=0 ." , then the correct answer in the box is N₀ .
But that's just a wild guess. As I pointed out, you cut off the picture in the middle of the word 'when', and I've got a hunch that there's something important after it.
