a. $\dfrac 13 - \dfrac14 = \dfrac{1{\color{Red}\times4}}{3{\color{Red}\times4}} - \dfrac{1{\color{Red}\times3}}{4{\color{Red}\times3}} =\dfrac 4{12}-\dfrac3{12} = \dfrac1{12}$.

b. $-\dfrac{11}{12}+\dfrac{25}{12}=\dfrac{-11+25}{12} = \dfrac{14}{12} =\dfrac{7{\color{Green}\times2}}{6{\color{Green}\times2}} = \dfrac76$.

c. $3 - \dfrac14 = \dfrac{3{\color{Red}\times4}}{\color{Red}4} - \dfrac14 =\dfrac{12}4 - \dfrac14 = \dfrac{11}4$.

d. $\dfrac{-3}{10} + \dfrac57 =\dfrac{-3{\color{Red}\times7}}{10{\color{Red}\times7}} + \dfrac{5{\color{Red}\times10}}{7{\color{Red}\times10}} =\dfrac{-21}{70}+\dfrac{50}{70}=\dfrac{29}{70}$.