notice how the second option, y=2x-1 is the only option that has a constant. All other options only have a coefficient on the x term. This makes it perfectly proportional by the factor of the coefficient.
100% = 1.00 is the cost of the jeans itself 6% = 0.06 is the sales tax to be added let "cost of jeans" be represented by x so you will get an expression (1.00 + 0.06) x + 8.95 = Total Cost
