The formula used to find the length of a line segment in space.

It's the same formula as the one used on Earth. It is based on the Pythagorean theorem.

length = √((∆x)^2 +(∆y)^2 +(∆z)^2)
.. where ∆x, ∆y, ∆z are the differences between the end-point coordinates in the x-, y-, and z-directions, respectively.

Y= 1/3x+2
Y= 1/3x-9

These lines are parallel because the slopes are the same that's the number with the X . They never touch

Y= -1/2x+8
Y= 2x+ 8

If the X has opposite signs and they are reciprocal of each other like -1/2 and 2/1 they are perpendicular. They cross (intersect) each other and make a 90 degree angle. Perfect L shape when they cross each other they are perpendicular