Answer
x$\approx$ 6.5
Work Step by Step
using theorem 11.4.3 Law of cosine
$x^{2}$ = $(10)^{2}$ + $(9)^{2}$ - 2*10*9*cos 40°
as we know cos 40° = 0.7660
$x^{2}$ = 100+81 - 180 * 0.7660
$x^{2}$ = 181 -180 * 0.7660
$x^{2}$ = 43.12
x = $\sqrt 43.12$
x$\approx$ 6.5
