What is the distance between #(7,9,4)# and #(3,-5,1)#?
I will let you finish this.
The image of strait line between these point can be projected onto the x,y plane. This, when considered in relation to the axis forms a triangle.
So you can determine the length of the projection on that plane by using Pythagoras.
The image on the xy plane is considered as the adjacent of a triangle and the z-axis as the opposite. Again you can use Pythagoras. This time the result is the actual magnitude of the distance between the points.
Let L be the length of the strait line between the points.
I will let you work that out!