12.
To find perimeter of triangles, use the formula P = a + b + c, where a, b, and c are the sides of the triangle.
For this triangle, you are given two sides and the perimeter and asked to find the third side. To do that, plug the given values into the formula for perimeter and solve.
P = a + b + c
10.8 = 3.5 + 2.6 + c
10.8 = 6.1 + c
4.7 = c
The measure of the third side of the triangle is 4.7 cm.
13.
The perimeter of a quadrilateral is the sum of the length of its four sides. So to find the perimeter, simply add all the sides together. As you need to find a missing side, follow the same steps as above to find the missing side.
15.3 = 2.7 + 4.7 + 3.3 + x
15.3 = 10.7 + x
4.6 = x
The length of the missing side of the quadrilateral is 4.6 cm.