Let $C: x^2+y^2=4$ and $C^{\prime}: x^2+y^2-4 \lambda x+9=0$ be two circles. If the set of all values of $\lambda$ so that the circles C and $\mathrm{C}^{\prime}$ intersect at two distinct points, is $\mathbf{R}-[a, b]$, then the point $(8 a+12,16 b-20)$ lies on the curve:
