\(\dfrac{12}{15}+x=-\dfrac{-3}{4}\\ \dfrac{4}{5}+x=\dfrac{3}{4}\\ x=\dfrac{3}{4}-\dfrac{4}{5}\\ x=\dfrac{3.5-4.4}{4.5}=\dfrac{15-16}{20}=-\dfrac{1}{20}\\ ---\\ x-\dfrac{-2}{3}=\dfrac{18}{26}\\ x+\dfrac{2}{3}=\dfrac{9}{13}\\ x=\dfrac{9}{13}-\dfrac{2}{3}\\ x=\dfrac{9.3-2.13}{3.13}=\dfrac{27-26}{39}=\dfrac{1}{39}\)
