Answer: The mistake is that david messed up on the inverse operation rules. He should have subtracted 2x, not add, and same goes to adding 2 to 33. He should have subtracted 2 not added it.
Explanation:
7x - 2 = 2x + 33
7x - 2x - 2 = 33
5x - 2 = 33
5x = 35
x = 7