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.

Answer 1

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.

Answer 2

P = 2L + 2W

768 <= 2L + (2 * 30)

768 <= 2L + 60

708 <= 2L

354 >= L