The value of x in the diagram is 19.6
Pythagoras theorem states that the sum squares of the two leg of a right triangle is equal to the square of the hypotenuse.
Mathematically;
c² = a² + b²
The large triangle is a right triangle and the hypotenuse is = 12 + 16 = 28
therefore;
28² = 20² + x²
x² = 28² - 20²
x² = 384
x = √ 384
x = 19.6
Therefore the value of x is 19.6