Do you know a program that can solve equatation by given variable? And where I can tell the program which variables are parameters?

take a look at this: http://www.sagemath.org/doc/tutorial/tour_algebra.html other possibilities which come to my mind: commercial: free: 


You could use WolframAlpha online (does an excellent job) or use Microsoft Mathematics (simply amazing). The best thing about Microsoft Mathematics is that, for many problems, it shows you the steps it took to solve the equation. They used to charge for Microsoft Mathematics, but it looks like it is free now. 


A simple one is Mathomatic, which is available both online and for download. 

