f(x) = 15lb/in2 + [(4.24lb/in2)/120in)x]
15lb/in2 is where the pressure starts at 0 inches below the surface.
Convert 10ft into inches = 120in
It changes 4.24lb/in2 every 120in. Divide by 120 to figure out how much it will change every 1in. Use this number as your slope.
Sorry if this isn't clear, but I hope it helps!
First of all, pounds per square inch is represented as psi, not lb/in^2.
Second of all, f(x) = 4.24x + 15.
Y is pressure in psi, X is every 10 feet of descent.