If it started to snow t hours after he started then he would have travelled 70t miles. It took him a further 10.8-t hours to get home travelling at 50mph, so he would have travelled 50(10.8-t) miles.
The whole distance is 630 miles, so 70t+50(10.8-t)=630.
Expanding the brackets: 70t+540-50t=630, 20t=630-540=90, t=90/20=4.5 hours.
It started to snow 4½ hours after he started his journey home.