#### Answer

γ = 90°

#### Work Step by Step

By using theorem 11.4.3
$c^{2}$ = $a^{2}$ + $b^{2}$ - 2ab cos γ
$5^{2}$ = $3^{2}$ + $4^{2}$ - 2*3*4* cos γ
25 = 9 + 16 - 24cos γ
25 = 25 - 24cos γ
25 - 25 = - 24cos γ
0 = - 24cos γ
cos γ = 0
γ = $cos^{-1}$(0)
γ = 90°