Notice that on step two, where equation 1 is simplifed, we have the following when we multiply by -2 the first equation:
[tex]\begin{gathered} -2\cdot(y=15-2z) \\ \Rightarrow-2y=-2(15)-2(-2z) \\ \Rightarrow-2y=-30+4z \end{gathered}[/tex]
therefore, the error is on the step 2
