Answer
$F \approx 33.7^{o}$
Work Step by Step
Apply law of cosines $f^2=d^2+e^2-2de \cos F$
Need to solve for $F$
$2de \cos x^{o}=d^2+e^2-f^2$
This implies $F=\cos^{-1}(\dfrac{d^2+e^2-f^2}{2de})$
Plug the given values to obtain:
$F=\cos^{-1}(\dfrac{(15)^2+(18)^2-(10)^2}{2 \times 15 \times 18}) \\=\cos^{-1}(\dfrac{449}{540})$
In order to calculate the value, we will use calculator in degree mode.
$F \approx 33.7^{o}$
