Teorema Pythagoras...

No.1 :
x = √(5² + 12²)
x = √(25 + 144)
x = √169
x = 13 cm

No.2 :
x = √(6² + 8²)
x = √(36 + 64)
x = √100
x =10 cm

No.3 :
x = √(20²- 12²)
x = √(400 - 144)
x = √256
x = 16 cm

No.1
x = √5² + 12²
x = √25 + 144
x = √169
x = 13

No.2
x = √8² + 6²
x = √64 + 36
x = √100
x = 10

No.3
x = √20² - 12²
x = √400 - 144
x = √256
x = 16