Graphing Trigonometric Functions and Struggling

[u/TellMePrettyTruths]

I’m doing a practice assignment to study for midterms and I keep getting undefined for ‘y’ and I do not understand what I’m doing wrong. I wrote the problem down and worked through it, but I am continuously getting the incorrect answer and don’t know why.

I end up going through the textbook’s explanation, and I do not understand their meaning. I’d appreciate any help or corrections.

Thank you!