Hello,

use Pythagorean theorem: a² + b² = c² with c as hypotenuse:

5² + 10² = 125 = c² ==> c = 5 sqrt(5)

Same method as above:

13² = 12² + b² ==> b² = 169 - 144 = 25 ==> b = 5

x is the number you are looking for:

(1/8) sqrt(7) - x = 2 ==> x = (1/8)sqrt(7) - 2 ≈ -1.66928...