##x^2+y^2=49##

The standard form of a circle with a center at ##(hk)## and a radius ##r## is

##(x-h)^2+(y-k)^2=r^2##

Since the center is ##(00)## and the radius is ##7## we know that

##{(h=0)(k=0)(r=7):}##

Thus the equation of the circle is

##(x-0)^2+(y-0)^2=7^2##

This simplifies to be

##x^2+y^2=49##

graph{(x^2+y^2-49)=0 [-16.02 16.03 -8.01 8.01]}