Chapter 9 - Quadratic Functions and Equations - 9-8 Systems of Linear and Quadratic Equations - Practice and Problem-Solving Exercises - Page 586: 32

$y=2.5x+30$

$y=.25x^2-5x+80$ $x=10$ $y=.25x^2-5x+80$ $y=.25*10^2-5*10+80$ $y=.25*100-50+80$ $y=25+30$ $y=55$ $(10,55)$ $x=20$ $y=.25x^2-5x+80$ $y=.25*20^2-5*20+80$ $y=.25*400-100+80$ $y=100-20$ $y=80$ $(20,80)$ $m=(80-55)/(20-10)$ $m=25/10 = 2.5$ $y=mx+b$ $55=10*2.5+b$ $55=25+b$ $55-25=25+b-25$ $30 = b$ $y=2.5x+30$