Solve for x :

cos $\dfrac{x}{3} - 1 = 0$

$\text{cos }\dfrac{x}{3} - 1 = 0$

$⇒ \text{cos } \dfrac{x}{3} = 1\\[1em] ⇒ \text{cos } \dfrac{x}{3} = \text{cos 0°}$

So,

$⇒ \dfrac{x}{3} = 0°\\[1em] ⇒ x = 3 \times 0°\\[1em] ⇒ x = 0°\\[1em]$

Hence, x = 0°.