This looks like a geometry problem, but it's also basically a function problem. They tell you a function (the measure of the angle) and then ask you to subtract the function with one input from the function with another input. Anyway.
Basically, just plug in 10 into the equation, and plug in 6 into the equation, and subtract the latter from the former. Let's go.
((10  2)x180)/10 = 8x18 = 144
((6  2)x180)/6 = 120
So 144  120 = 24. Done.
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