Bonsoir,
[tex]A=-1+x-y[/tex]
a.
[tex]A=-1+\dfrac{2}{3}-\left(-\dfrac{3}{4}\right)\\\\A=-1+\dfrac{2}{3}+\dfrac{3}{4}\\\\A=-\dfrac{12}{12}+\dfrac{8}{12}+\dfrac{9}{12}\\\\A=\dfrac{-12+8+9}{12}\\\\\boxed{A=\dfrac{5}{12}}[/tex]
b.
[tex]A=-1-\dfrac{1}{4}-\dfrac{5}{6}\\\\A=-\dfrac{12}{12}-\dfrac{3}{12}-\dfrac{10}{12}\\\\A=\dfrac{-12-3-10}{12}\\\\\boxed{A=-\dfrac{25}{12}}[/tex]