ahde

I have a 2004 LS430 fitted with standard 245/45R18 tires. My friend has offered me free two semi-new 235/45R17 tires with Lexus wheels taken off from his Lexus 2005 GS300. I am not sure if I can make good use of them to replace two of my older tires. Any advice pl.? If yes, should I put them at the front or rear? Thanks.