⑴2(18x)+18=36x+18

⑵2x(36x+18)+36x+18=72x*2+72x+18

⑶2x(72x*2+72x+18)+72x*2+72x+18=144x*3+216x*2+108x+18

(最後)
2x(144x*3+216x*2+108x+18)+144x*3+216x*2+108x+18=288x*4+576x*3+432x*2+144x+18


288x*4+576x*3+432x*2+144x+18=200
およそ61.5かな?