The surface area of hemisphere is given by:

Where r is radius of hemisphere.
We are given diameter = 250 ft.
We know that radius = half of diameter.
Radius of dome = 1/2 (250) = 125 ft.
Now let us plug the value of radius in the formula,

Surface Area of dome = 98125 square foot.
Factors of 6 = 1, 2, 3, 6
factors of 9 = 1, 3, 9
gcf = 3