Answer
$x^{o} \approx 27.0^{o}$
Work Step by Step
Apply law of cosines $c^2=a^2+b^2-2ab \cos C$
Need to solve for $C =x^{o}$
$2ab \cos x^{o}=a^2+b^2-c^2$
This implies $x^{o}=\cos^{-1}(\dfrac{a^2+b^2-c^2}{2ab})$
Plug the given values to obtain:
$x^{o}=\cos^{-1}(\dfrac{(17)^2+(28)^2-(15)^2}{2 \times 17 \times 28})$
$x^{o}=\cos^{-1}(\dfrac{848}{952})$
In order to calculate the value, we will use calculator in degree mode.
$x^{o} \approx 27.0^{o}$
