Subtract 15 from both sides:
[tex]\begin{gathered} -3\text{ - 15 }=\text{ 15 - 15 + 4y} \\ -3\text{ -15 = 4y} \end{gathered}[/tex][tex]\begin{gathered} -18\text{ = 4y} \\ \text{divide both sides y 4:} \\ \frac{-18}{4}=\frac{4y}{4} \end{gathered}[/tex][tex]\begin{gathered} -4\text{ }\frac{2}{4}\text{ = y} \\ y\text{ = }-4\text{ }\frac{1}{2} \\ y\text{ = -4.5} \end{gathered}[/tex]