Imagine that you were given a ship equipped with a device that could measure how deep the water is. you are asked to use this ship to find a subduction zone. what would you look for as you sailed around the world's oceans?

Deep ocean trenches

Explanation:

Subduction zones are Earth's lithosphere (the crust and as well as the upper non-convecting part of the upper mantle) gravitational collapsing sites. Along convergent plate boundaries, subduction zones exist where one sheet of oceanic lithosphere merges with any other layer. So, in order to find  depth of water in the sea, we can find these subduction zones. The best places are a deep ocean- trench like the Mariana Trench in the pacific ocean.

a and b sometimes have a positive value