We use a linear model

    y = mx + b


for the depreciated value, starting from x = 0 at 1991.



Therefore,  y = m*0 + b = 14520  in 1991, so we just know  b= 14520.



To find m, we write

    6240 = m*4 + 14520   for the year x= 4 (1995).



From this equation, we find the slope of the linear function

    m =  = -2070.



So, the final expression for the depreciated value linear function is

    y = -2070*x + 14520,  or  y = 14520 - 2070x.

Hi
x= 0 rpresenting 1991  and

 m = (14520-6240)/-4  = -8280/4 = -2070

  y = -$2070x + $14520

Wish You the Best in your Studies.