After you measured it you can check if the measurement is correct by knowing radius and arc length using formula,
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Hope this helps.
If you were to write it fully, as an equation, I believe it would look like this:
x²(4x-x) *4x
Therefore allowing you to simplify the area within the bracket into:
x²(3x)*4x
To expand, you simply start on the left and work your way across.
3x³*4x= 12x⁴
1/4(2x-14)=4
2x/4-14/4
X/2-7/2=4
x-7/2=4
8=x-7
8+7=x
X=15