The slope-intercept form of a line is y=mx+b where mm is the slope (the constant rate of change) and b is the yy-intercept.
The total cost per day depends on the number of miles driven so we need to let x be the number of miles and y be the total cost per day.
Since the company charges a fixed fee of $27 per day, then the yy-intercept of the line is b=27.
Since the constant rate of change is the fee of $0.79 per mile, then the slope of the line is m=0.79.
Substituting m=0.79 and b=27 into y=mx+b then gives y=0.27