So now I simply take the derivative of both sides and solve for y':
My answer book says different. I can't find a fault in my work. Here is what the book says:
Any help will be deeply appreciated. Thank you!
The fault is in the first line. The negative in red should be a positive. The derivative of sin x is cos x, NOT - cos x. This mistake has a ripple effect, easily fixed, which I'll let you work through.