Question 182170
Assume the two planes are heading 'towards P'. For now, let's not get too wound up about the x and y aspects. We'll always 'know' the two places are at right angles to each other.

First let's create two equations that represent the 'location' of each plane.
Plane1Location = 150 - 450(t)   (on the x axis)  
Place2Location = 200 - 450(t)   (on the y axis)

The distance between them ends up being the hypotenuse of the triangle with sides that are represented by Plane1Location and Place2Location.

{{{c^2 = a^2 + b^2}}}
{{{(c(t))^2 = (150-450t)^2 + (200-450t)^2}}}
Expand the right side, simplify and the take the square root to get the function c(t) over time.