#### Answer

0 miles

#### Work Step by Step

This problem can be solved using the formula given in the problem: Distance = \frac{7-x}{0.639x + 1.75}. Since the question asks about the distance of a river that has an altitude of 7000 ft, convert the 7000 ft into thousands of feet for the variable x by dividing 7000 by 1000. Therefore, x = 7.
Now, substitute 7 for x into the formula, which gives the expression:
\frac{7-7}{0.639(7) + 1.75}
Next, simplify the expression, and get the distance.
\frac{7-7}{0.639(7) + 1.75} = \frac{0}{4.473 + 1.75} = \frac{0}{6.223} = 0
As a result, the distance from the origin of the Nile when the river has an altitude of 7000 ft is 0 miles.