Ana walked 2 1/2 miles on the freedom trail. she walked 5/8 mile from boston common to the paul revere house. then, she walked 3/8 mile from the paul revere house to copp hill burying ground. how many more miles did ana walk to complete the trail to the uss constitution?

I have no idea what the total should be. What you have so far is

d = 2 1/2 miles + 5/8 miles + 3/8 miles
d = 5/2 + 5/8 + 3/8
The common denominator is 8.
To change 5/2 to something over 8, you have to multiply top and bottom of 5/2 by 4
5*4/2*4 = 20 / 8

d = 20/8 + 5/8 + 3/8
d = 28 / 8
d = 3 4/8
d = 3  1/2 miles