Answer
See answers below
Work Step by Step
Given: $ y = 0.0085x^2 - 1.5x + 120$
$y=0.0085(x^2-176.47x)+120$
$y=0.0085(x^2-2\times x\times88.235+7785.415)-66.176+120$
$y=0.0085(x-88.235)^2+53.824$
$y-53.824=0.0085(x-88.235)^2$
The vertex $(88.235,53.824)$ indicates the most efficient walking speed.