To complete the square for the expression 2 - 8x, we need to add a constant term that will make the expression a perfect square trinomial.
To do this, we can take half of the coefficient of x, square it, and then add it to the expression. In this case, the coefficient of x is -8, so:
(-8/2)^2 = 16
Therefore, we need to add 16 to the expression to complete the square:
2 - 8x + 16 = (2 - 8x + 16)
Simplifying the expression on the right-hand side:
2 - 8x + 16 = (18 - 8x)
So the number that should be added to complete the square of 2 - 8x is 16, and the completed square is (18 - 8x).