Word Problem?

Question

(1) The profit on a watch is given by P = x^2 -13x - 80 , where x is the nubmer of watches sold per day. How many watches were sold when there was a 50$ loss.

Is this how I set it up

P = (x, ) ( x, )

How would I solve this problem ?

Answer 1

P = x^2 - 13x - 80
We want to find x when P = -50.
x^2 - 13x - 80 = -50
=> x^2 - 13x - 30 = 0
=> (x-15) (x+2) = 0
=> x = 15 or -2, but x < 0 doesn't make sense.
So 15 watches were sold.

Answer 2

-50=x^2-13x-80

Now solve for x by setting to zero and factoring, or setting to zero and using the quadratic formula.