If they are going directly away from you, the problem is only one dimensional. Let us put you at $x=0$. Your enemy starts off at $x=5000$.
Your enemies position as a function of time is
$$ x_{enemy}=5000+400t $$
The rocket's position as a function of time is
$$ x_{rocket}=0+1500t $$
Set $x_{enemy}=x_{rocket}$ and solve for the $t$ at which they meet.
$$ 5000+400t=1500t\\\ 5000+400t-400t=1500t-400t\\\ 5000=1100t\\\ \frac{5000}{1100}=\frac{1100t}{1100}\\\ \frac{50}{11} = t\\\ $$
It takes $\frac{50}{11}=4\frac{6}{11}$ seconds.