We would have to solve the equation

Step by step to determine if Timothy made a mistake.
Step 1
Expand the bracket

Hence, his step 1 is correct.
Step 2
Bring like terms together

Step 3
Simplify step 2 and find the value of x

Hence, from all the steps through 1 to 3, we can conclude that Timothy did not make a mistake. Hence the right answer is option D