Chapter 6 - Inverse Circular Functions and Trigonometric Equations - Section 6.4 Equations Involving Inverse Trigonometric Functions - 6.4 Exercises - Page 285: 13

$x=\frac{1}{5}$ $arccos(\frac{-y}{2})$

$y=-2cos5x$ Need to solve $x$ in order to find this divide the above equation by -2 both sides. $-\frac{y}{2}=cos5x$ Apply definition of arccosine. $5x=arccos(\frac{-y}{2})$ $x=\frac{1}{5}$ $arccos(\frac{-y}{2})$