y=100 given x=<75 (less-than-equal-to) This should be obvious: it told you the cost would be flat-out total cost=y=$100 as long as no. of miles=x= under or equal to 75.
y=0.10(x-75)+100 given x>75
The second piece is because for every additional mile you drive, not only would you have to pay the $100 for the first 75 mile, you'd also have to pay for the additional no. of miles at an extra $0.10 per mile. The (x-75) is because: for example, if you drive 80 miles, you'd only be 80-75=5 miles over the 75-mile limit, so only 5 additional miles
