Answer
Error: use of a square of a binomial pattern instead of sum and difference pattern.
Correct result: $(s+5)(s-5)=s^{2}-25$
Work Step by Step
The error was to use the square of a binomial pattern instead of sum and difference pattern.
Recall: $(a+b)(a-b)=a^{2}-b^{2}$ (sum and difference pattern)
This implies that
$(s+5)(s-5)=s^{2}-5^{2}$
$=s^{2}-25$
