First we need to factor the left side. Since it is a perfect square (as is the process with completing the square, we know we can take half of the middle number along with x to be in the two parenthesis.
(x - 4)(x - 4) = 25
Now we simplify to show it as a square.
(x - 4)^2 = 25
Next we take the square root of both sides
x - 4 = +/- 5
Note that we have plus or minus 5. This is because either square would give us positive 25. Now we add 4 to both sides
x = 4 +/- 5
4 + 5 = 9
4 - 5 = -1
