Verified answer

Note that: cos(1/3 π) = 1/2

c² = a² + b² − 2ab ∙ cos(γ) =

16² + 8² − 2 ∙ 16 ∙ 8 cos(1/3 π) =

256 + 64 − 256 cos(1/3 π) =

320 − 256 cos(1/3 π) =

320 − 256 ∙ 1/2 =

320 − 128 =

192

But this is c² ! Therefore:

c = √(c²) = √192 = √64 ∙ √3 = 8√3

You don't ... cos(π/3) = 1/2, so just multiply all these terms together.

256 + 64 – 128 = 192

Unless I'm missing something in your notation ....

Hope that helps!