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
