For this type of question, you have to use the Pythagorean Theorem:

In the theorem, a and b are the legs. To find the length of the hypotenuse, you have to square the lengths of the two legs and add it together. Then, find the square root of the sum.
First square both numbers.


Now that you have the numbers, you need to add them.

Now you need to find the square root of 400 to find the length of the htypotenuse.

The length of the hypotenuse is 20 cm, which is the 3rd choice
a. 15 cm x
b. 17 cm x
c. 20 cm ✔
d. 28 cm x
If you have any questions, feel free to ask in the comments! :)