Answer:
4x - 16
Step-by-step explanation:
A triangle is a three sided polygon with three edges and three vertices. the sum of angles in a triangle is 180 degrees
the perimeter of a triangle = sum of the side lengths
let the unknown length be represented with y
13x - 21 = 4x + 3 + 5x - 8 + y
13x- 21 = 9x - 5 + y
Make y the subject of the formula
13x - 21 - 9x + 5 = y
4x - 16 = y
the side length = 4x - 16
