Write:
as
Now, try to make a common denominator en know that a fraction becomes zero if the numerator becomes zero, you have also to assure that r is not -10 or r is not 0, because in that case you're dividing by zero what's not defined.
EDIT:
I tried to solve the equation and 40 is definetly not the right answer, there're no real solutions.
Is there a typo in it? And also, if you enter 40 int the equation you get:
which is not true!
I think the equation has to be:
Yes, -50 and 40 are the solutions of the equation, but indeed it depends for which situation you're calculating this equation (if you have to reject a solution or not, you've to take in account a 'practical domain').