1/2-what=-1 3/4
$$\begin{array}{rcl}
\frac{1}{2} -x &=& -1 \frac{3}{4} \quad | \quad *(-1)\\\\
-\frac{1}{2}+x &=& 1 \frac{3}{4} \quad | \quad +\frac{1}{2}\\\\
x &=& 1 \frac{3}{4}+\frac{1}{2}\\\\
x &=& \frac{4*1+3}{4}+\frac{1*2}{2*2}\\\\
x &=& \frac{7}{4}+\frac{2}{4}\\\\
x &=& \frac{9}{4}\\\\
x &=& \frac{8+1}{4}\\\\
x &=& \frac{8}{4}+\frac{1}{4}\\\\
x &=& 2+\frac{1}{4}\\\\
x &=& 2\frac{1}{4}
\end{array}$$
1/2-what=-1 3/4
$$\begin{array}{rcl}
\frac{1}{2} -x &=& -1 \frac{3}{4} \quad | \quad *(-1)\\\\
-\frac{1}{2}+x &=& 1 \frac{3}{4} \quad | \quad +\frac{1}{2}\\\\
x &=& 1 \frac{3}{4}+\frac{1}{2}\\\\
x &=& \frac{4*1+3}{4}+\frac{1*2}{2*2}\\\\
x &=& \frac{7}{4}+\frac{2}{4}\\\\
x &=& \frac{9}{4}\\\\
x &=& \frac{8+1}{4}\\\\
x &=& \frac{8}{4}+\frac{1}{4}\\\\
x &=& 2+\frac{1}{4}\\\\
x &=& 2\frac{1}{4}
\end{array}$$