and$\displaystyle x^2+y^2-8x-4y-5=0$Deduce that the two circles touch each other. If the line of centres meets the circles in points P,Q,R, find the equation of the circle that has PR as diameter.$\displaystyle x^2+y^2+10x+20y+25=0$

r1+r2=15

sq root of (9^2+12^2) =15

therefore we can say they touch. but dont understand the second bit.

I don't know how to find the equation from the diameter of 15 from PR

PLease show step by step working!

Many thanks!