ANSWER
In step 3, he incorrectly wrote the square as (x + 4)². He should have written (x - 4)²
EXPLANATION
When he completed the square, he factored out 5 correctly. Then he found that the second term of the binomial was 4, so he added and subtracted the square of 4, which is 16. But in the next step, he made a mistake with the sign,
Instead of writing (x - 4)², he wrote (x + 4)².
