9a -3b= 5 -9a+12b=+24 adding both equations 9b=29, b=29/9
3a=4b-8=4*29/9 -8=(4*29-8*9)/9 = 44/9 a=44/27 a-b=44/27 - 29/9=44/27 - 29*3/27=-43/27 this is a correct answer, unfortunately they do not have this choice, probably they made mistake somewhere. I even check the solution of equations by substitution values of a and b into equations