Rule:

1.  Multiply the coefficient of x or y or whatever letter, by .
2.  Square the result of step (1).
3.  Add that to the right side.

First one:

a) x^2 - 8x

1.  Multiply the coefficient of x, which is (-8), by , 
getting  or 
2.  Square the result of step (1), getting , or 16.
3.  Add that to the right side.

Answer: x^2 - 8x + 16

You do the others.

b) x^2 - x

For that one the coefficient of x is (-1).

c) y^2 + 6cy

For that one, the coefficient of y is (+6c), (not just +6).

Edwin