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Bjr,
[tex]\dfrac{5}{a}=\dfrac{-3}{4}; \quad a\ne0\\\\ \dfrac{5\times 4}{a} = -3\\\\ 5\times 4 = -3a\\ 20 = -3a\\ -3a = 20\\\\ \boxed{a = \dfrac{-20}{3}}[/tex]
[tex]\dfrac{5}{a}=\dfrac{-3}{4}; \quad a\ne0\\\\ \dfrac{5\times 4}{a} = -3\\\\ 5\times 4 = -3a\\ 20 = -3a\\ -3a = 20\\\\ \boxed{a = \dfrac{-20}{3}}[/tex]