Consider this option:
1. if to re-write the condition, it is given: total time t=8 hours; upstream=downstream=6 miles; V_boat=4 m/h., V_current=V=?
2. note, that total time=time_upstream+time_downstream, where time_upstream=6miles/(V_boat-V) and time_downstream=6miles/(V_boat+V). Using this it is possible to make up and solve the following equation:

answer: √10
P.S. the roots of the equation are √10 and (-√10), only positive values is needed for V.
Answer:
= One-half more than three-fourths of a number
= Three-fourths minus one-half of a number
= three-fourths minus the sum of a number and one-half
Step-by-step explanation:
Suppose x be an unknown number.
∵ 3/4th of a number = 
= One-half more than three-fourths of a number
Now, half of a number = 
= Three-fourths minus one-half of a number
Sum of a number and one-half = 
= three-fourths minus the sum of a number and one-half
You do not have the options listed, but I will tell you what the line needs:
[] A parallel line will need the same slope.
- The slope is our m in y = mx + b
[] What will this be for our problem:
- 
- Our slope is 
[] So what will the line look like:
- 
- Since you do not have the options listed, I cannot tell you the answer, but I can tell you the answer will have a slope of 
Have a nice day!
I hope this is what you are looking for, but if not - comment! I will edit and update my answer accordingly. (ノ^∇^)
- Heather