See http://processing.org/reference/
There is log() and pow() and, of course, the base operations.

thank you :)

In case you may not know it, that divisor literal can be typed using scientific notation.  
-6.8755856 e-6

However, for the rest, it's better to store each part in separate variables. And go slow!

Also, I advise you to use the original Java's Math methods since they are double precision!
Processing has some wrappers for it, but use float instead.

Here's an attempt. Dunno whether it's correct though. But check it out:  

double dividend1, dividend2;
float  divisor1, divisor2;
float  PF = 12, PS = 35;
double Alt;

divisor1 = 5.2558797;
divisor2 = -6.8755856e-6;

dividend1 = Math.log10(PF/PS);

dividend2 = dividend1 / divisor1;
dividend2 = Math.pow(10, dividend2) - 1;

Alt = dividend2 / divisor2;
println(Alt);

exit();