Independent values are the things that change. Dependent values depend on the indepenent values. Independent is x and dependent is Y.
Did this come with multiple choice answers?
That trinomial can't be factored. You would have to complete the square to factor it and even then your answer would be an imaginary number.
x² + x + 12 = 0
x² + x = - 12
x² + x + 1/4 = - 12 + 1/4
x² + x + 1/4 = - 11 3/4
x² + x + 1/4 = - 47/4
(x + 1/2)² = - 47/4
x + 1/2 = +/- √(- 47/4)
x + 1/2 = +/-

x = 1/2 +/-

If the constant was a negative, it would be factor-able (is that a word?).
$3.45 is the cost of one gallon. So 3.45 times 12.51 gallons = 43.1595, rounded to $43.16