Find the equation of the circle with center at the origin that contains the point (-8,6)

The center is at origin O(0,0).
If it contains the point, P(-8,6), then the radius r is, by Pythagoras theorem,
r=sqrt((-8)^2+6^2)=10

The general equation of a circle at centre (xc,yc) with radius r is given by
(x-xc)^2+(y-yc)^2=r^2

Substituting r=10, (xc,yc)=(0,0)
the resulting equation is therefore

(x-0)^2+(y-0)^2=10^2
or simply
x^2+y^2=100