.
This sentence
"A rectangle's length and width are in a ratio of 19:6"
means that the length = 19x and the width = 6x, where x is the common measure.
Then for the perimeter you have
Length + width + length + width = 100, or
19x + 6x + 19x + 6x = 100,
====> 50x = 100 ====> x =
= 2 millimeters.
Then the length = 192 = 38 millimeters and the width = 6*2 = 12 millimeters.
Answer. The length and the width are 38 mm and 12 mm, respectively.
Solved.