Your class is just two functions wrapped inside a class declaration, and the first one
doesn't setup the motors AFAICT - it calculates some speeds and throws the results away.
The class should encapsulate the stuff, such as the four motors, which is it handles. The speeds
should be instance variables and would be passed to the motors when needed. Currently your motors
are global in scope and your class has no instance variables.
If you don't really understand what I've just said, get rid of the class, just use functions for now,
you'll have a much easier time understanding the compiler errors that way...