https://www.wolframalpha.com/input/?i=solve+a*(sin(x)+%2B+cos(x))+%2B+b*(+cos(y)+%2B+sin(y))+%2B+c*(+cos+(y-x))+%2B+d%3D+0+for+x
https://www.wolframalpha.com/input/?i=solve+a*(sin(x)+%2B+cos(x))+%2B+b*(+cos(y)+%2B+sin(y))+%2B+c*(+cos+(y-x))+%2B+d%3D+0+for+x