3x + 1 = 8 i need the steps and answer.

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?

During how many months was the difference between the number of days with homework and with no homework greater than 6?

What is the output, or dependent variable of quantity? 1: x 2: f(x) 3: y