there are several ways to approach this. here's one
note that the vertex is on the x-axis, that is, we have a double root for the vertex
thus, our quadratic is of the form $\displaystyle y = a(x - 3)^2$
we are given that the point (4,2) is on the graph, we can use this to find $\displaystyle a$, by plugging in $\displaystyle x = 4$ and $\displaystyle y = 2$ into the form above
what is your answer?