?? They tell you exactly how to go from 2.18 to 2.19: "Moving to the second row, we multiply by to obtain a leading entry of 1".
The matrix in 2.18 is .
Since the "reduced echelon" form that we are shooting for has "1"s on the diagonal, we need to change that " " to "1". We can do that by multiplying by its reciprocal , just as they said. Of course, we are doing "row operations" here so mutliplying that number by means multiplying every number in the second row by : , , , and giving 2.19: