A circle with its center on the line $y = 3x$ in the first quadrant is tangent to the $y$-axis.   

1. If the radius is $2$, what is the equation of the circle?

   Hint (a):

   What point on the line $y = 3x$ is $2$ units from the $y$-axis?

   Answer (a):

   Use the coordinates of the center and the radius to write an equation in general form.

   $(x - 2)^2 + (y - 6)^2 = 4$

2. If the radius is $3$, what is the equation of the circle?

   Hint (b):

   See part (a).

   Answer (b):

   $(x - 3)^2 + (y - 9)^2 = 9$

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