A truck is 30 km due west of an SUV and is travelling east at a constant speed of 70 km/h. Meanwhile, the SUV is going south at 90 km/h. When will the truck and the SUV be closest to each other?

I set up a triangle, defined dx/dt as -70t, dy/dt as 90t, and used Pythagorean theorem:

z^2 = x^2 + y^2

I then took the derivative of this.

I'm not sure where to go from here?