Question 73113
The profit is:
{{{p = x^2-13x-80}}}
Find x when p = ($50.00) The parentheses indicate a loss so this really should be written as -$50.00. 
So, you want to substitute p = -50 into the formula and solve for x, the number of watches.
{{{-50 = x^2-13x-80}}}  Add 50 to both sides and write the quadratic equation in standard form.
{{{x^2-13x-30 = 0}}} This can be factored.
{{{(x+2)(x-15) = 0}}} Apply the zero product principle.
{{{x+2 = 0}}} and/or {{{x-15 = 0}}}
{{{x = -2}}} Discard this solution as one cannot make a negative number of watches.
{{{x = 15}}} This is the solution.
15 watches were made on the day when there was a $50.00 loss.