Use Pythagoras theorem. The distance between the cars, 8km, is the hypotenuse of a right angled triangle, one leg of which is 6km and the other of which is, say, x km
The coefficient is the number just to the left of the variable. So that's why -8 is the coefficient for -8x. This is the variable term while 15 is the constant (it has no variable multiplied with it). The two terms are 15 and -8x since we can write 15-8x as 15+(-8x). Terms are separated by a plus sign.
