Try this solution:
1. the standart form of the circle is: (x-x₀)²+(y-y₀)²=r², where point (x₀;y₀) - the cetre of the given circle, r - radius of the given circle.
2. Using r=7 and coordinates of the centre (0;0) it is possible to make up the required equation:
(x-0)²+(y-0)²=7²;
x²+y²=49.
