The total ways to make sequences of 4 letters using the letters P, R, O, B, L, E, M, using each letter only once, without restrictions, is 7 * 6 * 5 * 4 = 840.
Let's use complementary counting for this problem (So, we find the situations we DON'T want and subtract them from the total)! :D
Ways that a sequence begins with L = 1 * 6 * 5 * 4 = 120
Ways that a sequence ends with P = 6 * 5 * 4 * 1 = 120
Ways that a sequence begins with L and ends with a P = 1 * 5 * 4 * 1 = 20
120 + 120 - 20 = 220
There are 220 ways which we DON'T want!
So, since the total is 840, we subtract 840 from 220 to get \(\fbox{620}\) :D
EDIT: Sorry I misread the problem! This answer is false.