Need the answer i don't know how to work it.

so.. what  is the coefficient "a"?
well
now, we know, another point on the graph, besides the vertex, we know a y-intercept, that is, 0, -30, that simply means when x = 0, y = -30



now, getting the x-intercepts, is just the zeros, or solution to the quadratic



notice, "y" is 0 on both cases, because, is an x-intercept, or a zero, and when the graph  touches the x-axis, "y" is zero

When i did the math i got 11 cm