Hi

If you know the formula of the distance between a point and a line it is very easy since the radius of the circle is the distance between the center C and the tangent line (D)

The formula for a point A(xA,yA) and a line (d) ax+by+c=0 is

If you don't know the formula then you can use the point of tangence H with the conditions

(i) H is on the line (D)

(ii) (CH) is perpendicular to (D)