Lindsay finds the perimeter of a figure when a side length is not given. Practice this yourself on Khan Academy right now: https://www.khanacademy.org/math/cc-third-grade-math/cc-third-grade-measurement/cc-third-grade-perimeter/e/find-a-missing-side-length-when-given-perimeter Watch the next lesson: https://www.khanacademy.org/math/cc-third-grade-math/cc-third-grade-measurement/cc-third-grade-compare-area-perimeter/v/comparing-area-and-perimeter Missed the previous lesson? https://www.khanacademy.org/math/cc-third-grade-math/cc-third-grade-measurement/cc-third-grade-perimeter/v/perimeter-of-a-shape 3rd grade math on Khan Academy: We know you've been rocking through 2nd grade adding and subtracting all kinds of whole numbers (up to 2 digits, right?). That's awesome! In 3rd grade math we want you to start using bigger numbers and start multiplying and dividing, too. By the way, did you know that some numbers aren’t actually “whole?” They’re “partially whole.” We call them fractions!