2z + x = 11.5

You didn't tell us what to do with it.  Always give
the instructions when you post a problem, so we will 
know what to do with it.

Were the instructions "Solve for x"?  If so then

2z + x = 11.5

Subtract 2z from both sides:

     x = 11.5 - 2z

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Were the instructions "Solve for z"? If so then

2z + x = 11.5

Subtract x from both sides:

    2z = 11.5 - x

Divide both sides by 2

    2z     11.5 - x
   ---- = -----------
     2         2

    1
    2z     11.5 - x
   ---- = -----------
     2         2
     1 

           11.5 - x
      z = -----------
               2

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Were the instruction "Graph the equation"?  If so,
   z



Edwin