Answer
$r=\dfrac {1}{10};a_{1}=5000$
Work Step by Step
$\dfrac {a_{m}}{a_{n}}=r^{m-n}\Rightarrow \dfrac {a_{7}}{a_{3}}=r^{4}=\dfrac {0.005}{50}=\dfrac {1}{10^{4}}\Rightarrow r=\dfrac {1}{10};a_{3}=a_{1}\times r^{2}\Rightarrow 50=a_{1}\times \left( \dfrac {1}{10}\right) ^{2}\Rightarrow a_{1}=5000$