radius = 7
the equation of a circle in standard form is
(x-a)² + (y-b)² = r²
where (a, b) are the coordinates of the centre and r is the radius
To obtain this form ' complete the square ' on both the x and y terms
x² + 2(1)x + 1 + y² + 2(3)y + 9 = 39 + 1 + 9
(x+1)² + (y+3)² = 49 ← in standard form
with centre (1, 3) and r = √49 = 7
