where (h, k) is the center of the hyperbola.
From your asymptote equations y = 3x and y = -3x it is clear that the center is C(0, 0). So your hyperbola has the form:
What is the relationship between a and b? (Look at the general equation for the asymptotes.)
You have a point V(0, 6) on the hyperbola. How can you use that?