Question 388157
This is a "right triangle" problem solved using pythagorean theorem. 
The theorem is:
{{{a^2 + b^2 = c^2}}}

Where a and b are lengths of the two shorter sides of a right triangle, and c is the length of that triangle's longest side.

We know this is a right triangle because the problem statement says so. The only shape that going north (think going up the Y axis of a chart) followed by going west (think going down the X axis in the negative direction) could be is a right triangle, and the two distances mark the two shorter sides of the triangle:

g<------12------^
                |
                |
                7
                |
                h

So if the trip starts at h and ends at g, the length of g->h is the longest side of this triangle, i.e. c.
a = 7
b = 12
{{{a^2 = 7^2 = 49}}}
{{{b^2 = 12^2 = 144}}}
{{{49 + 144 = 193 = c^2}}}
{{{c = sqrt(193)}}} which is about 13.89 miles.