step 1
In the smaller right triangle of the figure (triangle BCD)
Applying Pythagorean Theorem
see the attached figure to better understand the problem
BC^2=BD^2+DC^2
substitute given values
BC^2=x^2+4^2
BC^2=x^2+16 ------> equation A
In the right triangle ABD
Applying Pythagorean Theorem
AB^2=x^2+16^2
AB^2=x^2+256 ------> equation B
In the right triangle ABC
Applying Pythagorean Theorem
20^2=AB^2+BC^2 -------> equation C
substitute equation A and equation B in equation C
20^2=x^2+256+x^2+16
400=2x^2+272
2x^2=400-272
2x^2=128
x^2=64
