Answer:
Step 3 is not correct (should be  , not
, not 
Step-by-step explanation:
The expression that we have to simplify is:

We proceed step-by-step, in order to find the mistake did by Cedric:
1) First, we factorize the 5 outside:
 --> this step is correct
 --> this step is correct
2) We add +16 and -16 inside the brackets:
 --> this step is correct
 --> this step is correct
3) We rewrite the term  as the square of a bynomial, which is
 as the square of a bynomial, which is  , so the expression becomes
, so the expression becomes
 --> we notice that this step is wrong: in fact, Cedric wrote
 --> we notice that this step is wrong: in fact, Cedric wrote  , which is not correct.
, which is not correct.
4) Now we continue: we rewrite -16+3 as -13,

5) Finally, we multiply the 5 by the terms in the bracket:
