First, you would add 3/4+3/4 to see how many miles he walks each day. To get this you would simply add the numerators because the denominators are the same. This would get you to 3/4+3/4=6/4 You can then turn this fraction into a mixed number (1 2/4) which can be simplified into 1 1/2. He walks 1 1/2 miles one day and the second day he walks 1 1/2 miles. Add the two days together to get that in 2 days he walks 3 miles.
The final answer is 2 days. I hope I helped :)