Question 1158349
The function I(t) = -0.11 + 1.6t represents the yearly income=> I have noticed you have linear equation; there is no certain point where a line decline

I assume you have:

 {{{I(t) = -0.1t^2 + 1.6t }}} which is a parabola that have a maximum at vertex

so, write equation in vertex form: 



{{{I(t) = (-0.1t^2 + 1.6t) }}}....factor out {{{-0.1}}}

{{{I(t) = -0.1(t^2- 16t) }}}........complete square

{{{I(t) = -0.1(t^2- 16t+b^2)-(-0.1)b^2 }}}.....{{{b=16/2=8}}}

{{{I(t) = -0.1(t^2- 16t+8^2)+0.1*8^2 }}}

{{{I(t) = -0.1(t- 8)^2+0.1*64 }}}

{{{I(t) = -0.1(t- 8)^2+6.4 }}}

=> {{{h=8}}} and {{{k=6.4}}}=> vertex is at ({{{8}}},{{{6.4}}})

a maximum will be at {{{8}}} years, and after {{{8}}} years income will begin to decline


{{{drawing ( 600, 600, -10, 15, -10, 15, 
circle(8,6.4,.12),locate(8,6.4,V(8,6.4)),
graph( 600, 600, -10, 15, -10, 15, -0.1(x- 8)^2+6.4)) }}}