Old school math teachers are basically drill seargants, teaching you to use a hammer for one nail type of problem, right swing technique, and never to question the other ideas concerning the hammer.
I went a bit further in math afterward so I could at least grasp at the essentials of calculus, but that one moment took a lot of the wind out of my sails.
If you have a figure like this, you write this, then this, then "according to Thales' theorem", then that result.
No, you didn't write "according to Thales' theorem", the result is wrong. Write it again.
No, you can't write it in this equivalent way, you have to write it as taught. Write it again.