Highlighted parts are my own style of completing the square. I personally don't recommend my students to memorize interpreting the radius and centre of circle from the general form (which was given in this question).

For the second part, I highly recommend you to draw a rough sketch to visualize what they are asking for. And take note that in coordinate geometry, product of 2 lines that are perpendicular to each other = -1.
i.e. m1 m2 = -1.

Equation of line is y - y1 = m ( x - x1), where (x1, y1) is the coordinates of the point that is involved.