The area of any circle is (pi) x (radius squared).
The radius is of the table 1/2 of the diameter . . . That's 6 feet.
If the table were a full circle, its area would be (pi) x (6 squared) = 36 pi square feet.
But it's only half of that . . . 18 pi = (18) x (3.14) = 56.52 square feet.
Is that enough help ?