Find the range of a function f(x)=-5x-1 for the domain {-5,1,7}

How to do this , and can some one explain

Find the constant of variation for the relation and use it to write an equation for the statement. then solve the equation.

At a kentucky friend chicken, a survey showed 2/3 of all people preferred skinless chicken over the regular chicken. if 2,400 people responded to the survey, how many people preferred regular chicken?