area for circle is πr² so πr² =28.26 and we can sub 3.14 as pi
3.14*r²=28.26 we can divide by 3.14 to get r² on it's own
r²=28.26/3.14
then we root both sides to get r on it's own
28.26/3.14=9 √9=3
and the diameter is double the radius 3*2=6 so the diameter is 6