What is 8.57 as a fraction or a mixed number

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

This is so hard and it’s for a test

Which shows the correct substitution of the values a,b and c from the equation 1=-2x+3x^2+1