The problem is:
Given two boats, one, the coast-guard boat, stands on the point zero, the thief's boat, on the point $D$, the coast-guard boat travels in a speed of $Vg$ knots and the thief's boat, in $Vf$ knots, Given $D$, $Vf$ and $Vg$, say if it's possible to the coast guard boat reach the thiefs boat before the thief's boat reach $12$ miles away from the point $0$
I've started with an Inequalitie $0 + (i*Vg) <= D + (i*Vf)$ with $D + (i*Vf) < 12 miles$ being $i$ a moment. but know I'm stuck on how to continue, and how to get a formula to solve it.
Sorry for the inconvenience, I've just misunderstood the statement, they aren't perpenticular to the cost when they leave, imagine as a cartesian plane, the coast-guard boat is in the point $(0,0)$ and the thief, in the point $(d, 0)$ and the objective of the thief is reach the point $(d, 12)$ before the coast guard boat reach him, it gives an all new view to to problem, that's why my example in the comments isn't wrong..
Sorry for the incovenience.