ANSWER
5 miles
EXPLANATION
Let's draw the two possible paths Bob can take. One is walking 3 miles south and 4 miles west, and the other is walking in a straight line from where he is to the park,
If Bob takes the red path, the total distance he will travel is 7 miles. If he takes the blue path, the total distance he will travel can be found with the Pythagorean Theorem,
[tex]c^2=a^2+b^2[/tex]
Solve for c taking the square root of both sides,
[tex]c=\sqrt{a^2+b^2}[/tex]
Replace a and b, and solve,
[tex]c=\sqrt{3^2mi^2+4^2mi^2}=\sqrt{9mi^2+16mi^2}=\sqrt{25mi^2}=\sqrt{25}mi=5mi[/tex]
Hence, the distance Bob will walk if he takes the blue path is 5 miles, which is the shortest path.