Evaluate D(40) and interpret the result

Question

I am stuck on this question for my homework I know that it is giving me all the info but I feel like it is a puzzle and I am missing a piece the question is:

 The federal debt D in trillions of dollars

held by foreign and international investors x

years after 1970 can be modeled by

D(x) =1/320(x^2)-3/80(x) where 15<x<40

where 15 … x … 40.

(a) Evaluate D(40) and interpret the result.

(b) Determine the year when the federal debt held

by foreign and international investors first

reached $500 billion ($0.5 trillion).

 

Thank You in Advance for you help.

Answer 1

D=(1/320)x^2 -(3/80)x, x=yeer+1970, x<=40

=(0.003125)x^2 -0.0375x

for x=40...(0.003125*1600 -0.375*40=5 -1.5=3.5 trilyun $

#### part 2...yeer debt=0.5 trilyun

(0.003125)x^2 -0.0375x -0.5=0

me put it intu quadratik equashun & get roots=-8, or 20...yuze 20