The work of a student to solve a set of equations is shown: equation 1: m = 8 + 2n equation 2: 6m = 4 + 4n step 1: −6(m) =   −6(8 + 2n)       [equation 1 is multiplied by −6.] 6m =   4 + 4n       [equation 2] step 2: −6m =   −48 − 12n       [equation 1 in step 1 is simplified.] 6m =   4 + 4n       [equation 2] step 3:   −6m + 6m =   −48 − 12n + 4n       [equations in step 2 are added.] step 4: 0 =   −48 − 8n step 5: n =   −6 in which step did the student first make an error? step 3 step 5 step 4 step 2

The answer is:  "Step 3" . 
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Note:  "Step 3"  incorrectly shows:   "−6m + 6m =  −48 − 12n + 4n " .
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{Rather than the correct equation; which is:

   " - 6m + 6m =  - 48 – 12n + 4 + 4n " .}.
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Note of interest:
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Although not asked in the question/problem, let us continue with the correct equation; & to solve for" n ; 
 

" - 6m + 6m =  - 48 – 12n + (4 + 4n) ; 

     →  " 0 = - 48 – 12n + 4 + 4n " ; 
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Combine the "like terms" on the "right hand side of the equation:
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            -48 + 4 = -44 ;

           - 12n + 4n = - 8n ; 
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  → Rewrite the equation:  " 0 = -8n – 44 " ; 

            ↔  " - 8n – 44 = 0 " ; 

Add "44" to each side of the equation; 
  
            →    -8n – 44 + 44 = 0 + 44 ; 

to get:
 
            →    -8n  = 44 ; 

 Now, divide  EACH SIDE of the equation by:  "-8 " ;
  to isolate "n" on one side of the equation; & to solve for "n" ;  

            →    -8n / 8 = 44 / 8 ;

            →     n = 44/8 = (44 ÷ 4) / (8÷4) = 11/2 ;  

n =  ""  ; or, write as:  "5 "  ;  
                                           
                                           or, write as:  " 5.5 " .
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