Need-math-help-for-finding-a-linear-function

The idea of a large, stable middle class (defined as those with annual household incomes in 2010 between $39,000 and $118,000 for a family of three), is central to America’s sense of itself. But the U.S. middle class has been shrinking steadily from 61% of all adults in 1971 

(
t = 0)

 to 51% in 2011 

(
t = 4),

 where 
t is measured in decades. Research has shown that this decline is approximately linear.
†
Find a linear function 
f(
t) giving the percentage of middle-income adults in decade 
t, where t = 0 corresponds to 1971.