The triangle is a right triangle, so the Pythagorean theorem applies.
It says, for this triangle,
[tex]14.7^2=11.9^2+x^2[/tex]
Solve for [tex]x[/tex]; you'll find
[tex]x^2=14.7^2-11.9^2[/tex]
[tex]x=\pm\sqrt{14.7^2-11.9^2}[/tex]
(but you omit the negative square root, since [tex]x[/tex] must be a positive length)
Use a calculator to find an approximate value:
[tex]x=\sqrt{14.7^2-11.9^2}\approx8.64[/tex]
(measured in km)
