How do you do the quadratic equation when there are only two numbers, such as -5x and 15, not the three needed?
-5x + 15=0
a=0 b=-5 c=15
Now, because the a is in the denominator, you can't have zero. So the quadratic formula wont work.
-5x=-15
x=3
So you solve it another way, usually by factoring.