Is this what you are trying to do:
f := (8 x + 2)/7 - (12 x - 3)/5 == -1
Solve[f, x]
Hello,
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To reply to what you asked, here is an example. Notice the difference between = (Set) which sets the value of a variable, and == (Equal) which asserts equality. (There are also ways of producing a more typeset expression.)
In[1]:= equation = (8 x + 2)/7 - (12 x - 3)/5 == 1
Out[1]= 1/5 (3 - 12 x) + 1/7 (2 + 8 x) == 1
In[2]:= solution = Solve[equation, x][[1]]
Out[2]= {x -> -(1/11)}
In[3]:= x /. solution
Out[3]= -(1/11)
