The circle C C\,C has centre (5,k)(5, k)(5,k), where k k\,k is a constant. The line y=2x+1y = 2x + 1y=2x+1 is a tangent to the circle CCC, touching C C\,C at the point A(3,7)A(3, 7)A(3,7).
Find an equation for CCC.