let the northbound ship start at (0,0)

let the eastbound ship start at (0,15)

northbound ship's position (x,y) as a function of time, t , in hrs ...

(0,12t)

eastbound ship's position (x,y) as a function of time ...

(9t,15)

use the distance formula to set up an expression for the distance between the two ships as a function of time ... take the time derivative and minimize.