Two cars leave an intersection. One car travels north; the other east. When the car traveling north had gone 12 miles, the distance between the cars was 8 miles more than the distance traveled by the car heading east. How far had the east bound car traveled?

I thought the equation to be 8+B+B-12 the answer being 2miles

You didn't consider the direction. If they were travelling in opposite directions, then you would have done what you have written.

Use Pythagoras' Theorem for this problem, which states that the square of the hypotenuse in a right-angled triangle is the sum of the square of the two sides.

The distance car A travelled is 12 km.
Let the one car B travelled be B km.
Their distance apart then is (B + 8) km.

If you have made a sketch, you'll see that (B+8) is the hypotenuse.

I guess you can complete it now. Post what you did and your answer!