The maximum speed of the ball can be found fairly easily, since we already have a lot of the information needed. We know that the maximum speed occurs just before the ball hits the ground, because gravity continues to accelerate it until then, and therefore it will be going the fastest right before that point. This means that in order to find the maximum speed, we must take the horizontal velocity, which is constant, and the final vertical velocity, which we found before (see Velocity & Components), and apply Pythagorean Theorem to find the actual final velocity of the ball, which will be the maximum speed:
Therefore the maximum speed is 17.8m/s