Your first solution ($$-4y + 16 = X$$) is the same as theirs, they just chose to leave the negative sign on the X. Your second solution has a slight sign error -- you move the 16 over to the right and then the X to the left (here it is done with both their form of the equation and yours): \begin{align} 4y - 16 &= -x & -4y + 16 &= x \\4y + x &= 16 & -4y - x &= -16 \\x + 4y &= 16 & 4y + x &= 16 \end{align}