Question 133091
Given: C(x)=2x^2+70x+812  is the model. And $2312 is spent.

{{{2312 = 2x^2 + 70x +812}}}
{{{0 = 2x^2 + 70x - 1500}}}
{{{ 0 = x^2 + 35x - 750}}}
{{{ 0 = (x - 15) (x + 50) }}}
Set each factor equal to 0, yeilds x=15 or x=-50
Since one cannot feed -50 customers, the only reasonable answer is 15 customers.

Check you answer.
{{{C(x)=2x^2+70x+812}}}
{{{C(x)=2(15)^2+70(15)+812}}}
{{{C(x)=450 + 1050 + 812}}}  Check!