Let f(x) = x square + bx + c > 0 for every value of x in r or f(x)< 0, for every value of x in r which of the following is correct

There is a linear relationship between a salesperson sales and her weekly income.if her sales are $200 her income is $500 and if her sales are 1,200 her income is $600.what is the relationship between sales and income