The form of the equation of a circle is r2=(x-a)2+(y-b)2, where r is the radius and (a,b) is the center.
The length of the line PQ is the diamiter of the circle, so plug (-1,2) and (7,8) into the distance formula
\(d=\sqrt{(-1-7)^2+(2-8)^2}\)
\(d=\sqrt{(-8)^2+(-6)^2}\)
\(d=\sqrt{64+36}\)
\(d=\sqrt{100}\)
\(d=10\)
so the radius is 5
The midpoint of the line PQ is the center of the circle, so plug P and Q into the midpoint formula
\(m=(\frac{-1+7}{2},\frac{8+2}{2})\)
\(m=(\frac{6}{2},\frac{10}{2})\)
m=(3,5)
Plugging the midpoint and the radius into the equation for the circle gets:
25=(x-3)2+(y-5)2