Maths discriminant question HELP PLEASE

Hello,
OK theres a equation x^2 -6kx +9k^2
prove x axis tangent to curve

well discriminant = 0, that show the curve touching x axis

But
Since the discriminant is zero for all values of k, the x-axis is always a tangent to the (curve equation up top)

Ok i don't understand you solve for k from b^2-4ac = 0,
gives 36k^2 - 36k^2=0
gives k=0
I don't understand solving the k part, what does it resemble?
How it relates to context of question too?