#### Answer

$-2$

#### Work Step by Step

$\left( \dfrac {5}{2}\right) ^{x}=\dfrac {4}{25}\Rightarrow \left( \dfrac {5}{2}\right) ^{x}=\left( \dfrac {2}{5}\right) ^{2}\Rightarrow \left( \dfrac {5}{2}\right) ^{x}=\left( \dfrac {5}{2}\right) ^{-2}\Rightarrow x=-2$