Linear Equations

a) For solving this equation, we simply have to divide both sides by 4: \begin{eqnarray}4x = 20 && |:4\\ x = 5\end{eqnarray} b) Here, we have two options. Either, we expand the left side or we divide both sides by 3. Let us solve this equation by using the expansion method: \begin{eqnarray}15x - 21 = 9 && |+21 \\ 15x = 30 &&|:15 \\ x = 2\end{eqnarray} If we first divide the equation by 3 and then solve it for $x$, we will of course get the same result.