What GoldenRivet says is true though. The tools you have on board, TDC, maybe you have attack disk as well, etc. are designed to help you solve the equations without doing the math. I posted just so that you could see the math because I like to know how things work. I'm a bit geeky like that.
If you want the true course because you're working on plotting an intercept, parallel course, or something the math could help, or you could chart it, or use the attack disk. Attack disk is the fastest method IMHO and has several uses as exampled in the
Angriffsscheibe Handbuch. It also shows the formulae for the calculations.