Question

While skateboarding at 19 km/h, Alana throws a tennis ball at 11 km/h to her friend Oliver. If Alana is the reference frame, the speed of the tennis ball is

Answer 1

Well, you see, I don't think Alana really is the reference frame. 
Call me paranoid, but I think you changed the reference frame
during the question, and didn't tell us.

In which reference frame is the 19 km/hr measured ?
It CAN'T be Alana's reference frame.  Your own reference
frame moves along with you, and you can't move in it, even
if your name is Alana.

If Alana is the reference frame, and she throws the tennis ball
at 11 km/hr, then the speed of the ball is 11 km/hr in Alana's
reference frame.  Her reference frame moves with  her, so
it makes no difference how fast she is skateboarding in any
other reference frame, who she throws the ball at, or whether
or not he sees it coming and catches it.

Yes, this stuff can get confusing.  And if you think it's bad now,
wait till you start reading some of Prof. Einstein's stuff, where
two people in the same reference frame can watch the same
tennis ball, and not even agree on how fast it's moving, because
THEY're both moving and their own motion makes their rulers
and clocks change !  So they measure different speeds, and
they're both right !

But I got distracted.  I'm sorry.  The point I'm trying to make,
right now when you're just starting to learn reference frames,
is that EVERY time you say a speed, you have to tell which
reference frame the speed is in.  Because, as you're starting
to learn, the same object can have a different speed in every
reference frame.

And, just in case you're thinking about this later today
and you want to ask "Which one is the REAL speed  ?" . . .
THAT's the whole point of learning about reference frames !
There is NO SUCH THING as REAL speed.  It ALWAYS
depends on which reference frame it's measured in.  They're
all different, they're all real, and they're all correct.

Answer 2

The first thing we must do for this case is to define a frame of reference.

We know that, the frame of reference for this case is Alana.

Therefore, the tennis ball has a speed relative to Alana.

The speed relative to Alana, is the same speed with which Alana throws the tennis ball.

Therefore, the speed of the tennis ball is:

v = 11 Km/h

Answer:

The speed of the tennis ball is:

v = 11 Km/h