$$ \begin{aligned} -50 &= 6a+28&& \text{ swap the left and right side of the equation } \\[1 em]6a+28 &= -50&& \text{ move 28 to the right } \\[1 em]6a &= -50-28&& \\[1 em]6a &= -78&& \text{ divide both sides by $ 6 $ } \\[1 em]a &= -\frac{78}{6}&& \\[1 em]a &= -13&& \\[1 em] \end{aligned} $$