Explanation:
this is totally simple.
the formula for the area of a circle (as you can find with any search on the internet in seconds) :
A = pi×r²
r is the radius.
and once you understand even the most basics of a circle, you know that the radius is half of the diameter.
so, in our case, r = 6/2 = 3 m
the area of the circle is therefore
pi×r² = pi×3² = pi×9 m²