Please explain how to do

You would have to find the area of the largest circle and the areas of the two smaller triangles (in this case you could just find one of the areas and multiply them by two since they have the same diameter!). After finding the areas add the smaller areas together and subtract it from the area of the larger circle!
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(Circle of Area is pi*(radius^2))
(Diameter = 2*radius)
(Pi=3.14)

Gray Area = Large Circle Area - 2(Smaller Circle Area)

Gray Area = (pi*8^2) - 2(pi*2^2)
Gray Area = 64pi - 2(4pi)
Gray Area = 64*3.14 - 2(4*3.14)

Remember to round to the one decimal place!