What is the positive value of k for which the graph is $y=x^2 -2kx+16$ is tangent to the $x$-axis? 

My approach to solving this is that since the parabola is tangent to the $x$-axis, I can find the value of $x$ by using $-\frac{b}{2a}$. Once I have $x$ I can solve for $k$. Is this the right approach?