Question 972435
The tangent to the circle meets on the circle for a line with slope {{{-1/2}}} and goes through (0,0).  Center of the circle is at (0,0).  {{{y=-x/2}}} must intersect {{{y=2x+c}}} for a point ON the circle.  The intersection point will be 6 units from the origin, because the radius of the circle is 6.  Make use of the Distance Formula.  That is the general idea although I have not worked through to actually try to solve.