2800
The math behind it:
800(Mph) x 3(Hours) = 2400(Miles traveled in 3hrs)
800(Mph) x 1/2(or .50hrs) = 400(Miles traveled in 1/2 an hr)
2400(Miles traveled in 3hrs) + 400(Miles traveled in 1/2 an hr) = 2800(Miles traveled in 3 and 1/2hrs)
Hope this helps! :) Have a nice day!