The given equation which determines the percent of people in favor of capital punishment is:
y = -1859 + 0.9753x
where x starts from 1975.
Consider that the slope of the line is 0.9753. It is also the rate of change of the percent of people in favor with the capital punsihment per year.
Then, you can conclude that the slope of the linear model means:
Each year, the percent that favors capital punishment increases by 0.9753% on average.