Answer: The length of the third straw is 29 centimeters.
Step-by-step explanation: Given that Anja is creating a triangle using three straws and the lengths of two shorter straws are 20 cm and 21 cm.
We are to find the length of the third straw in centimeters so that Anja will create a right triangle.
The two shorter straws will make the legs of the right-angled triangle because they are shorter.
Since the third straw will be the largest, so it will make the HYPOTENUSE of the triangle.
In a right-angled triangle, we have
Hypotenuse² = length of one leg² + length of the other leg².
Therefore, we get
Thus, the length of the third straw is 29 centimeters.