If $\displaystyle 3 \pi < \theta < \frac{7 \pi}{2}$, what is $\displaystyle arccos(sin\theta)$ equal to?

Attempt:

Since $\displaystyle 3 \pi < \theta < \frac{7 \pi}{2}$, $\displaystyle \theta = \frac{\pi}{2}$ so:

$\displaystyle arccos(1)$

$\displaystyle = 0$

but think I made a mistake without considering the interval it in. What should I do