Question

Simon is riding a bike at 12 km/h away from his friend Keesha.He throws a ball at 5 km/h back to Keesha, who is standing still on a sidewalk.
 
How fast would Keesha say the ball is traveling toward her?

Answer 1

I notice that even though we're working with frames of reference
here, you never said which frame the '5 km/hr' is measured in.

In fact !  You didn't even say which frame the '12 km/hr' of his
bike is measured in.

So there are several different ways this could go.  I'll do it the way
I THINK you meant it, but that doesn't guarantee anything.

-- Simon is riding his bike at 12 km/hr relative to the sidewalk,
away from Keesha.

-- He throws a ball at Keesha, at 5 km/hr relative to his own face.

-- Keesha sees the ball approaching her at (12 - 5) = 7 km/hr
relative to the ground and to her.