Chapter 2 Linear Equations and Functions - 2.7 Use Absolute Value Functions and Transformations - 2.7 Exercises - Skill Practice - Page 127: 15

$y=-3|x|$.

The functions have the form $y=a|x-b|+c$ where $(b,c)$ is the vertex of the function. Hence, since the vertex is at $(0,0)$, $y=a|x|$. Since $(1,-3)$ is on the function, plugging in the values we get: $-3=a|1||\\-3=a$. Thus $y=-3|x|$.