Let r be the radius of the circle, which touches x - axis at point (a, 0), a<0 and the parabola y^2=9 x at the point (4,6). Then r is equal to ____

Let $\displaystyle r$ be the radius of the circle, which touches $\displaystyle x$ - axis at point $\displaystyle (a, 0), a<0$ and the parabola $\displaystyle y^2=9 x$ at the point $\displaystyle (4,6)$. Then $\displaystyle r$ is equal to $\displaystyle \_\_\_\_$