A word problem is given which I've answered correctly, but a second question doesn't add up. The problem is:

A North-South highway intersects an East-West highway at a point P. An automobile crosses P at 10:00 AM, traveling east at a constant rate of 20 mph. At that same instant another automobile is two miles north of P, traveling south at 50 mph. Find a formula which expresses the distance d between the automobiles at time t (hours) after 10:00 AM. At what time will the automobiles be 104 miles apart?

My answer, which the book confirms, is

To answer the second question, I started with

Using the Quadratic Formula, I get

which I can only reduce to

But the answer given in the book is:

So how do I get there? Did I make a mistake along the way?