Find the slope of the line using the two points by plugging them into this formula: m = y2 - y1 / x2 - x1 where the two points are x1, y1 and x2, y2 and m is the slope.
Basically you just need to multiply the two fractions together to get the depth of the turtle. Therefore the turtle is 3/12 miles deep or 1/4 mile deep.