to find symmetrical point on the line, you should first do distance of that point on the line ....

if your line is :

and you have some point at

to find distance between line and point you can do like this (or you can use formula, but let's do it like this so you understand what and how is done)

Let's first mark one point at the line as and on start of the line vector we put that point (now look at the picture shown, than continue)

distance from the point and line is equal to ratio of surface of the parallelogram constructed on vectors and and intensity of the vector

as shown :

or in scalar form

can you figure the rest ? i'll go now bye