13·x² + 13·y² = 49

It's a circle because x² and y² have the same coefficient when they
are on the same side of the equation.

Divide through by 13

 +  = 

 +  = 


x² + y² =   in the form x² + y² = r²

That's a circle with center at the origin and radius = 

That's a weird value for a radius.  It's like 1.941450687

But here's the graph:



Edwin