a-3*-1/2-1/2=0

$$\begin{array}{rll}
a-3\times \left(\frac{-1}{2}\right)-\frac{1}{2}&=&0\\\\
a&=&3\times \left(\frac{-1}{2}\right)+\frac{1}{2}\\\\
a&=&\left(\frac{-3}{2}\right)+\frac{1}{2}\\\\
a&=&\frac{-3+1}{2}\\\\
a&=&\frac{-2}{2}\\\\
a&=&\;-1


\end{array}$$

.