(I'm from Brazil)
Welcome! Give the syntax a try .... first one defines the function f
In[231]:= Clear[f, x]
f[x_] := 6 x - 2
then one uses, runs or calls it, giving the argument as usual, note the brackets!
In[233]:= f[2]
Out[233]= 10
In[234]:= f /@ Range[-3, 3]
Out[234]= {-20, -14, -8, -2, 4, 10, 16}