Hi

solving by the addition-or-subtraction method: 

 12a-5b=18 and 

 15a-5b=30



  12a-5b=  18  

 -15a+5b= -30  |mulitply 2nd EQ by -1 to eliminate b

  -3a = -12

    a = 4  and b = 6     (b = (15*4-30)/5 = 6



CHECKING our Answer***

   12a-5b=18

    48-30 = 18 

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