there are infinitely many possible values:
2(30+L) >= 768
30+L >= 384
L >= 354
so, any number at least 354 will work.
I suspect you did not ask the right question.
The width of a rectangle is 30 centimeters. Find all possible values for the length of the rectangle if the perimeter is at least 768 centimeters.
2 answers
P = 2L + 2W
768 <= 2L + (2 * 30)
768 <= 2L + 60
708 <= 2L
354 >= L
768 <= 2L + (2 * 30)
768 <= 2L + 60
708 <= 2L
354 >= L