Question

which trig function do you use? cosine tangent sine

Explanation:

Step1: Recall trigonometric function definitions

In a right - triangle, $\cos\theta=\frac{\text{adjacent}}{\text{hypotenuse}}$.

Step2: Identify sides relative to the given angle

The given angle is $40^{\circ}$, the hypotenuse is $19$ and $x$ is adjacent to the $40^{\circ}$ angle. So, $\cos40^{\circ}=\frac{x}{19}$.

Answer:

Cosine