Question: intersecting planes

Hello everybody. My lovely Dutch math book is progressing. Last week ive been scanning a copy of Advanced Engineering Mathematics from the library, a grueling task which took me 24 full hours to complete. But ive got a book that should be able to teach it with the right mindset and social contract (this Dutch book is too distracting, i cant even think). I will finish it though.. The sturdy that stands i guess. 

Any way the problem here is that the side from which a line interfaces with a shape in the 3D plane only has an x component and is lacking a y and z component. So the calculations are impossilbe, because you cant devide through 0. I suspect its something simple, but like said the repertoire is so terrible it feels like im scrambled eggs if i even remotely think about solving the problem. The facade makes the man right? Feels more like one of these spagetti westerns with only a frontage without anything behind it. 

The Question is as follows (translated from Dutch): "Given is the half ball with the equation z=sqrt(4-x^2-y^2). Check that the given point lays on the half ball and determin the eqation of the interface with the half ball in the given point."

So basically have a vector from that given point that will hit the surface of the half ball.   
 

Thank you very much. The first answer "a" was right, but for the other ones, this method they gave does not work. 

Greetings,

The Function 

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