Check the picture below.
the peak height is reached when the firework reaches its maximum speed, because at that point it begins to drop to the ground, and that occurs as with any other parabolic path, at its vertex as you see in the picture, at 3.8 seconds.
Now, the maximum hmmm this will just be an estimate because the graph is only showing elapsed time and speed, we know it reached at maximum speed of 9.2 m/s at 3.8 seconds after it was ignited, so an estimate will be the product of those two, at a speed of 9.2 m/s for a lapse of 3.8 seconds that'd give use (9.2)(3.8) = 34.96 metres or thereabouts.