Chapter 10 - Geometry - 10.3 Polygons, Perimeter, and Tessellations - Exercise Set 10.3 - Page 636: 19

perimeter = 18 yards

RECALL: (1) The perimeter of a polygon is equal to the sum of the lengths of all its sides. (2) An equilateral triangle has three side with equal lengths. This means that the two unknown side lengths of the triangle are both $6$ yards. Add the lengths of all the sides to find the perimeter: perimeter = $6+6+6= 18$ yards