Answer
$140^{\circ}$
Work Step by Step
First, ensure that your calculator is in radians.
On your graphing calculator, press the following sequence of buttons:
1. COS$^{-1}$
2. (
3. -.766
4. )
5. ENTER
Therefore, $cos^{-1}(-.766)=2.443$ radians. Then, convert this number to degrees by multiplying by $\frac{180}{\pi}$.
$2.443radians=(2.443(\frac{180}{\pi}))^{\circ}=140^{\circ}$.