Take a look at the right triangle. The vertical leg has length 12 cm because the square is 12 cm on a side.
According to the Pyth. Thm., x^2 + (12 cm)^2 = (20 cm)^2. Then:
x^2 = 400 cm^2 - 144 cm^2, or x^2 = 56
Then the unknown side length, x, is +√56 = +7√8 = +14√2.
