Answer:
I guess, that's an Associative Property
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The central angle COA = 2 * angle at the circumference
m < COA = 2 * 80 degrees
So you reversed the subtraction a bit; the equation you're gonna be solving is

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First step ya gonna do is to reverse the + and - symbols in the trinomial thats in the parentheses so your equation looks like

Now just combine like terms and your answer should be
.19 seconds
This is because 12.38-12.19= .19