You want to move all numbers on the same side of x to the other side with y. Whenever doing this, we have to reverse the operation. In this problem, we have + 3 with x. So we will need to subtract 3 from both sides.
A perpendicular line makes a angle of 90° with a line or a plane.
If a line is to be drawn from a point to a line or a plain it can only be one. In this case a line is to be drawn through a point A to a plane P. If the line is to be perpendicular, then is is only one.