Go from Oceanfront 5 units left to Landview. Now go up to Seaside 6 units up.
From Oceanfront to Seaside it's the same as the hypotenuse of the triangle made by the path shown above. 5^2 + 6^2 = 25 + 36 = 61
I know 8^2 = 64, so I estimate the square root of 61 to be about 8.7 units on the graph.
We are told each unit on the graph is 3 miles. 8.7 * 3 is approximately 26.
Answer: 26 miles