Every instruction before executing will take 3 clock cycles per word as given in the question.
$\\ I1: 3*2+3=9\\ I2: 3*1+3=6\\ I3: 3*1+1=4\\ I4: 3*1+2=5\\ I5: 3*2+3=9\\ I6: 3*1+0=3\\ \\ Total=36 Cycles\\ \\ cycle time=1/clock frequency=1/2GHz=0.5ns\\ \\ Therefore\ total\ time=36*0.5ns=18ns$