They want you to combine the variables. So −12x4 + 7x3 + 6x4 − 2x3 = <span>5x3 − 6x4 </span>So you combine the coefficients, not the variable number, for instance -12+6=-6, and then you simply put the variable in the same place it was. You don't do anything else with it except what I just showed you : ) <span />