How do i find the center & radius, then write a standard equation for a, b, and c?

Firstly , we will locate centres and radius in the figures

(a)

We can see that

centre=(h,k)=(0,0)

radius=r=5

now, we can use standard equation of circle

we can plug values

we get

(b)

We can see that

centre=(h,k)=(5,4)

radius=r=3

now, we can use standard equation of circle

we can plug values

we get

(c)

We can see that

centre=(h,k)=(4,3)

radius=r=5

now, we can use standard equation of circle

we can plug values

we get