a ball is thrown into the air with an initial velocity of 30m\s and a horizontal velocity of 10m\s. What's it's speed after 4 seconds?

Answer 1

The horizontal velocity remains unchanged, but the vertical changes.

In the vertical
vf=30-9.8t
find the vertical velocity, add it to the horizontal as VECTORS.

Answer 2

.....huh? that would make the vertical velocity negative

Answer 3

of course: negative means downward.