Astore advertised 15% off an idem that regularly sells for 300

The polynomial function of least degree with a leading coefficient of 1 is: f(x) = x3 + ax2 + bx + c

Cassie is traveling 675 miles today to visit her brother and his family. so far, she has stopped for gas three times. at the first stop, she put in 7.1 gallons of gas and paid $23.79. at her second stop, she put in 5.8 gallons of gas and paid $19.08. and, at the third stop, 6.6 gallons and paid $20.39. at which stop did casey pay the most for gas?

How do you solving the solution of each system using elimination?