I placed the givin equation on the claculator, I am assuming that you are looking at a circle? There for I used the negative cosine which is the second function and then cos button on the calculator. Then place the fraction 10/15 into the calculator, you can simplify to 2/3 if wanted to. After you plug it in the caluculator you get the arc measurement, I got 48.189685104221° after I plugged in the equation into the calculator. Hope that helped some what
