Associative property of addition is The order of the parentheses. There are no parentheses is this problem. We are using the commutative property of addition which states we can change the order of the addends
A^2 + b^2 = c^2...a and b are the legs and c is the hypotenuse 20^2 + 21^2 = c^2 400 + 441 = c^2 841 = c^2 sqrt 841 = c 29 = c <== third straw will be 29 cm