Answer
$y\approx15.82$
Work Step by Step
$A=45^{\circ},s=18,r=15$
For arc-length,
$s=r\cdot C$
$18=15\cdot C$
$C=\frac{18}{15} radian$
$C=68.76^{\circ}$
Using laws of sines,
$\frac{r}{sinA}=\frac{y}{sinC}$
$\frac{15}{sin45^{\circ}}=\frac{y}{sin68.76^{\circ}}$
$y\approx15.82$
