How do you solve Thevenin equivalent for circuits with variable resistors?
The problem is not to have one (or two) variable resistors, because you always take a look at one moment and treat them as fixed during your analysis.

I guess your problem is to see that the variable resistor (potentiometer) actually consists of two resistors (R2 and R - R2). In general they form a  voltage divider; in this particular case one of the two "sub"-resistors (R - R2) is shorted so you can forget the (R - R2) part (i.e. treat it as being 0).