solve for the missing angle.

Explanation:

Step1: Recall cosine - inverse formula

Let the missing angle be $\theta$. We know that $\cos\theta=\frac{\text{adjacent}}{\text{hypotenuse}}$. Here, the adjacent side to the angle $\theta$ is 13 and the hypotenuse is 18. So, $\cos\theta=\frac{13}{18}$.

Step2: Calculate the angle

$\theta = \cos^{- 1}(\frac{13}{18})\approx43.3^{\circ}\approx44^{\circ}$

Answer:

$44^{\circ}$