Author 
Comment/Response 
Bill Simpson

10/04/12 12:26pm
Solve for variables a and c, you must use == and not =
In[1]:= Solve[{a+1==3b, 9b3==48c},{a,c}]
Out[1]= {{a> 1+3b, c> 1/16(1+3b)}}
Find value of a/c from solution of Solve
In[2]:= ReplaceAll[a/c,Solve[{a+1==3b, 9b3==48c},{a,c}]]
Out[2]= {16}
It is also sometimes possible to eliminate one or more variables from a set of equations.
In[3]:= Eliminate[{a+1==3b, 9b3==48c},b]
Out[3]= 16 c==a
Study the help pages for Solve, ReplaceAll, Eliminate and look at the examples to try to see how to use these for other problems.
Mathematica by itself will not show any of the steps used in a calculation. WolframAlpha.com will sometimes show steps used in a calculation if you see a "Step by Step Solution" to click on. You can also ask Mathematica to send a question to WolframAlpha for solution, but the web page may be easier to use.
Note: the commands used and the things that Mathematica and WolframAlpha assume can be very different and you should be very careful to check results to make certain they are correct.
Mathematica is FANATIC about correct use of [] versus () versus {} and correct capitalization and correct use of = versus := versus == and there is even an === and those are all completely different. Make any tiny mistake with any one of those and you will get incorrect results or error messages you may not understand or even get nothing at all.
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