Answer
$$ -\frac{1}{2}(\cos^{-1}t)^2+c.$$
Work Step by Step
Since $ u=\cos^{-1}t $, then $ du=\frac{-1}{\sqrt{1-t^2}}dt $.
Now, we have
$$\int \frac{\cos^{-1}t}{\sqrt{1-t^2}}dt =-\int udu\\
=-\frac{1}{2}u^2+c=-\frac{1}{2}(\cos^{-1}t)^2+c.$$