Aside from what Ishuda told you, the only advice I have is to try and write explicitly what you mean. For instance, at first I couldn't tell what the two theta equals statements in red meant. I would have written something like:
θ∈[0∘,360∘]
Additionally, when first taking the derivative, you wrote dy/dx. Taking the derivative of y with respect to x would result in 0, as there is no x in the expression. Now, it might seem like I'm just nitpicking and being overly critical, but establishing these practices will help you immensely later on. When you start working with more complex equations with more than two variables and/or using u-substitution, being clear about what variables you're derivating or integrating with respect to is
very important. If you're not careful, you can end up with a totally wrong answer because of poor notation.