I see that you set up 90 miles = 60 mph * t, where you found out that t = 1.5. What what you needed to do was set 180 miles = 45 mph * t. This is because it is 45 mph for the <em>whole</em> trip. Then, you find that t = 4 hours. That that means that he spent 2.5 hours driving back. Then, we can do d=vt and find 90/2.5 as the speed he went home at. Which comes out to be 36 mph.
