First you should subtract 13 from -8.
Then divide the answer by -3.
Because you are dividing by a negative number, reverse the sign and you will get your answer.
13 - 3n < -8
-8 - 13 = ?
-3n < ?
?/-3 = n
Like in this example,
3 - 5n < 13
13 - 3 = 10
-5n < 10
10/-5 = -2
n > -2
