Write the number with the lest value using the digit 1 through 9. use each digit 1 through 9.use each digit only once. it do tomorrow i need

In order for the function to be linear, what must m be and why? m = 22 because the rate of change is 1. m = 20 because the rate of change is 3. m = 22 because the rate of change is -1. m = 20 because the rate of change is -3.