You will use the Pythagorean Theorem to find the length of the walkway.
A diagonal across creates 2 congruent triangles which are right triangles.
a^2 + b^2 = c^2
12^2 + 9^2 = c^2
144 + 81 = c^2
225 = c^2; find the square root of 225
c = 15 meters
The length of the walkway is 15 meters.