if we think about it, the touching parts of the rectangle and circle are the corners
the distance between opposite corners is the diameter of the circle
the radius would be 1/2 diameter
so we just use the pythagorean theorem to find the legnth of the diagonal and thus the diameter then halve that to get radtius
a^2+b^2=c^2
c=hypotonuse
a and b are legs
8^2+6^2=c^2
64+36=c^2
100=c^2
10=c
10=diameter
10/2=5=radis
5 centimeters