$\frac{-5}{12}$ + $\frac{1}{-4}$
= $\frac{-5}{12}$ + $\frac{-1}{4}$
= $\frac{-5}{12}$ + $\frac{-3}{12}$
= $\frac{-8}{12}$ = $\frac{-2}{3}$
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$\frac{-7}{15}$ + $\frac{3}{35}$
= $\frac{-49}{105}$ + $\frac{9}{105}$
= $\frac{-40}{105}$ = $\frac{-8}{21}$
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$\frac{-5}{7}$ + $\frac{-3}{4}$
= $\frac{-20}{28}$ + $\frac{-21}{28}$
= $\frac{-41}{28}$
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$\frac{5}{12}$ + $\frac{-3}{28}$
= $\frac{35}{84}$ + $\frac{-9}{84}$
= $\frac{26}{84}$ = $\frac{13}{42}$