Use the fact that

(x-h)² + (y-k)² = r² is the equation of a circle,

where (h,k) = the center and r = the radius.

In your problem,

origin = (0,0) and so h=0 and k=0  and radius = r = 7.

(x-0)² + (y-0)² = 7²

which simplifies to

x² + y² = 49

Edwin