A simple confusion: Showing projectile is launched horizontally

[Faiq]

The equation of trajectory of ball B is y = -0.05x^2 where y is vertical displacement and x is horizontal. Prove that the ball is projected horizontallyFor this, the mark scheme says
x tan a = 0 so a =0
Since the angle between ball and surface is 0 the object is projected horizontally.
I can't work out how is x tan a = 0

 

[mfb]

The problem statement got lost somehow.

 

[Faiq]

Please check again

 

[mfb]

I don't see where the x would come from.

tan a is the derivative of your function at x=0. The derivative is 0, therefore tan a = 0.

 

[Faiq]

How do you know the derivative is 0?

 

[Faiq]

Got it thanks