Answer
The recommended safe distance for the given speed is $124$ feet
Work Step by Step
The formula that models the recommended safe distance is
$$S=0.015x^{2}+x+10$$
The speed given is $60$ miles per hour.
Substitute $x$ by $60$ in the formula given to obtain the recommended safe distance when traveling at the given speed:
$S=0.015(60)^{2}+60+10=...$
$...=0.015(3600)+60+10=...$
$...=54+60+10=124$ feet
The recommended safe distance for the given speed is $124$ feet
