Because of the perpendicular, you should be able to draw this as a right triangle. The two legs are 2 miles (p=plane) and 4 miles (m=missile). The hypotenuse is given by Pythagoras' theorem. The hypotenuse is the distance between the plane and missile.
h2 = p2 + m2
You are asked for the instantaneous rate of change of h, that is, dh/dt.
So take the derivative of both sides:
d/dt (h2) = d/dt (p2 + m2)
If you work carefully and apply the chain rule where you need to, you will get an expression with dh/dt in it (you should solve for this, since that is what you are trying to find), h, m, p (all of which you know or can calculate from the pythagorean theorem), and dm/dt and dp/dt which are the speeds of the missile and plane. Plug in the values and find the value of dh/dt.
Hope this helps,
Stephen La Rocque.