To solve the presented question for X, every change we make in one of the sides of the equality, we must do the same to the other. This way, the equality remains. In the present case, we will subtract 15.09 in both sides, as it follows:
[tex]\begin{gathered} x+15.09=-2.57 \\ x+15.09-15.09=-2.57-15.09 \\ x=-17.66 \end{gathered}[/tex]