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put this solution on YOUR website!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.

Expand the right side, simplify and the take the square root to get the function c(t) over time.