y - value
x - year
[tex]24000=a\cdot3+b\\
19000=a\cdot5+b\\\\
24000=3a+b\\
-19000=-5a-b\\
---------\\
5000=-2a\\
a=-2500\\\\
b+3\cdot(-2500)=24000\\
b-7500=24000\\
b=31500\\\\
\boxed{y=-2500x+31500}
[/tex]
a)
[tex]f(x)=-2500x+31500\\
f(x+1)=-2500(x+1)+31500\\
f(x+1)=-2500x-2500+31500\\
f(x+1)=-2500x+29000\\\\
f(x)-f(x+1)=-2500x+31500-(-2500x+29000)\\
f(x)-f(x+1)=-2500x+31500+2500x-29000\\
f(x)-f(x+1)=\boxed{2500}[/tex]
b)
[tex]f(0)=-2500\cdot0+31500=\boxed{31500}[/tex]
c)
[tex]f(10)=-2500\cdot10+31500\\
f(10)=-25000+31500\\
f(10)=\boxed{6500}[/tex]